feat(audio): aggressive preloading and job queue foundation

Improve audio caching with more aggressive preloading:
- Increase cache size from 25 to 50 sentences for better buffering
- Remove setTimeout delays - start all preloads immediately
- Increase preload ahead count from 3 to 5 sentences
- Add 500ms recovery delay when skipping problematic sentences
- Better error logging with sentence previews

Add foundation for background audiobook generation:
- Create Job type definitions for audiobook generation tasks
- Add jobs table to IndexedDB (database version 6)
- Implement job CRUD operations in Dexie
- Track job status, progress, and results
- Support for querying jobs by status

This prepares for server-side background audiobook processing
that doesn't require keeping the browser open.

Changes:
- TTSContext: Increase cache to 50, remove preload delays, add error recovery
- types/jobs.ts: New file with job type definitions
- lib/dexie.ts: Add jobs table and helper functions

src/contexts/TTSContext.tsx

@@ -296,7 +296,7 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
 
   // Audio and voice management hooks
   const audioContext = useAudioContext();
-  const audioCache = useAudioCache(25);
+  const audioCache = useAudioCache(50); // Increased from 25 to 50 for better buffering
   const { availableVoices, fetchVoices } = useVoiceManagement(openApiKey, openApiBaseUrl, configTTSProvider, configTTSModel);
 
   // Add ref for location change handler
@@ -1068,7 +1068,10 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
               // User won't be interrupted by error toasts during reading
               console.warn('Skipping problematic sentence, continuing playback...');
 
-              advance();
+              // Add a small delay before advancing to give the server a brief rest
+              setTimeout(() => {
+                advance();
+              }, 500); // 500ms delay before trying next sentence
             }
           },
           onend: function (this: Howl) {
@@ -1123,7 +1126,10 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
       // Just log to console and move to next sentence
       console.warn('Skipping problematic sentence, continuing playback...');
 
-      advance();
+      // Add a small delay before advancing to give the server a brief rest
+      setTimeout(() => {
+        advance();
+      }, 500); // 500ms delay before trying next sentence
       return null;
     }
   }, [isPlaying, advance, activeHowl, processSentence, audioSpeed]);
@@ -1201,15 +1207,16 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
   }, [activeHowl, isPlaying, currentSentenceAlignment]);
 
   /**
-   * Preloads the next 3 sentences' audio for smoother playback
-   * Uses progressive delays to avoid overwhelming the server
+   * Preloads the next 5 sentences' audio for smoother playback
+   * Starts all preloads immediately for maximum buffering
    */
   const preloadNextAudio = useCallback(async () => {
     try {
-      // Preload the next 3 sentences to ensure smooth playback
-      const PRELOAD_AHEAD_COUNT = 3;
-      const PRELOAD_DELAY_MS = 50; // Small delay between requests
+      // Preload the next 5 sentences to ensure smooth playback
+      const PRELOAD_AHEAD_COUNT = 5;
 
+      // Start all preloads immediately (no delays)
+      // The server-side cache and de-duplication will handle load balancing
       for (let i = 1; i <= PRELOAD_AHEAD_COUNT; i++) {
         const nextSentence = sentences[currentIndex + i];
         if (!nextSentence) break; // Stop if we've reached the end
@@ -1222,17 +1229,13 @@ export function TTSProvider({ children }: { children: ReactNode }): ReactElement
           ttsModel,
         );
 
+        // Only start preload if not already cached or in progress
         if (!audioCache.has(nextKey) && !preloadRequests.current.has(nextSentence)) {
-          // Add progressive delay to avoid overwhelming the server
-          // First request: 50ms, second: 100ms, third: 150ms
-          const delay = PRELOAD_DELAY_MS * i;
-
-          setTimeout(() => {
-            // Start preloading but don't wait for it to complete
-            processSentence(nextSentence, true).catch(error => {
-              console.warn(`Error preloading sentence ${i} ahead:`, error);
-            });
-          }, delay);
+          // Start preloading immediately (no setTimeout delay)
+          // Fire and forget - errors are logged but don't block playback
+          processSentence(nextSentence, true).catch(error => {
+            console.warn(`Error preloading sentence ${i} ahead (${nextSentence.substring(0, 30)}...):`, error);
+          });
         }
       }
     } catch (error) {

src/lib/dexie.ts

@@ -1,10 +1,11 @@
 import Dexie, { type EntityTable } from 'dexie';
 import { APP_CONFIG_DEFAULTS, type ViewType, type SavedVoices, type AppConfigRow } from '@/types/config';
 import { PDFDocument, EPUBDocument, HTMLDocument, DocumentListState, SyncedDocument } from '@/types/documents';
+import { Job } from '@/types/jobs';
 
 const DB_NAME = 'openreader-db';
 // Managed via Dexie (version bumped from the original manual IndexedDB)
-const DB_VERSION = 5;
+const DB_VERSION = 6; // Bumped to 6 for jobs table
 
 const PDF_TABLE = 'pdf-documents' as const;
 const EPUB_TABLE = 'epub-documents' as const;
@@ -12,6 +13,7 @@ const HTML_TABLE = 'html-documents' as const;
 const CONFIG_TABLE = 'config' as const;
 const APP_CONFIG_TABLE = 'app-config' as const;
 const LAST_LOCATION_TABLE = 'last-locations' as const;
+const JOBS_TABLE = 'jobs' as const;
 
 export interface LastLocationRow {
   docId: string;
@@ -30,6 +32,7 @@ type OpenReaderDB = Dexie & {
   [CONFIG_TABLE]: EntityTable<ConfigRow, 'key'>;
   [APP_CONFIG_TABLE]: EntityTable<AppConfigRow, 'id'>;
   [LAST_LOCATION_TABLE]: EntityTable<LastLocationRow, 'docId'>;
+  [JOBS_TABLE]: EntityTable<Job, 'id'>;
 };
 
 export const db = new Dexie(DB_NAME) as OpenReaderDB;
@@ -154,12 +157,14 @@ function buildAppConfigFromRaw(raw: RawConfigMap): AppConfigRow {
 
 // Version 5: introduce app-config and last-locations tables, migrate scattered config keys,
 // and drop the legacy config table in a single upgrade step.
+// Version 6: add jobs table for background audiobook generation
 db.version(DB_VERSION).stores({
   [PDF_TABLE]: 'id, type, name, lastModified, size, folderId',
   [EPUB_TABLE]: 'id, type, name, lastModified, size, folderId',
   [HTML_TABLE]: 'id, type, name, lastModified, size, folderId',
   [APP_CONFIG_TABLE]: 'id',
   [LAST_LOCATION_TABLE]: 'docId',
+  [JOBS_TABLE]: 'id, status, type, createdAt',
   // `null` here means: drop the old 'config' table after upgrade runs,
   // but Dexie still lets us read it inside the upgrade transaction.
   [CONFIG_TABLE]: null,
@@ -555,3 +560,56 @@ export async function loadDocumentsFromServer(
 
   return { lastSync: Date.now() };
 }
+
+// Job management helpers
+
+export async function createJob(job: Job): Promise<void> {
+  await withDB(async () => {
+    console.log('Creating job via Dexie:', job.id);
+    await db[JOBS_TABLE].put(job);
+  });
+}
+
+export async function getJob(id: string): Promise<Job | undefined> {
+  return withDB(async () => {
+    console.log('Fetching job via Dexie:', id);
+    return db[JOBS_TABLE].get(id);
+  });
+}
+
+export async function getAllJobs(): Promise<Job[]> {
+  return withDB(async () => {
+    console.log('Fetching all jobs via Dexie');
+    return db[JOBS_TABLE].orderBy('createdAt').reverse().toArray();
+  });
+}
+
+export async function getJobsByStatus(status: string): Promise<Job[]> {
+  return withDB(async () => {
+    console.log('Fetching jobs by status via Dexie:', status);
+    return db[JOBS_TABLE].where('status').equals(status).toArray();
+  });
+}
+
+export async function updateJob(id: string, updates: Partial<Job>): Promise<void> {
+  await withDB(async () => {
+    console.log('Updating job via Dexie:', id);
+    await db[JOBS_TABLE].update(id, updates);
+  });
+}
+
+export async function deleteJob(id: string): Promise<void> {
+  await withDB(async () => {
+    console.log('Deleting job via Dexie:', id);
+    await db[JOBS_TABLE].delete(id);
+  });
+}
+
+export async function clearCompletedJobs(): Promise<void> {
+  await withDB(async () => {
+    console.log('Clearing completed jobs via Dexie');
+    const completedJobs = await db[JOBS_TABLE].where('status').anyOf('completed', 'failed', 'cancelled').toArray();
+    const ids = completedJobs.map(job => job.id);
+    await db[JOBS_TABLE].bulkDelete(ids);
+  });
+}

src/types/jobs.ts

@@ -0,0 +1,46 @@
+/**
+ * Background job types for audiobook generation
+ */
+
+export type JobStatus = 'pending' | 'processing' | 'completed' | 'failed' | 'cancelled';
+
+export type JobType = 'audiobook-generation';
+
+export interface AudiobookJobData {
+  documentId: string;
+  documentName: string;
+  voice: string;
+  speed: number;
+  audioPlayerSpeed: number;
+  ttsProvider: string;
+  ttsModel: string;
+  ttsInstructions?: string;
+  format: 'mp3' | 'm4b';
+}
+
+export interface Job {
+  id: string;
+  type: JobType;
+  status: JobStatus;
+  data: AudiobookJobData;
+  progress: number; // 0-100
+  currentStep?: string; // Human-readable status
+  error?: string;
+  result?: {
+    bookId: string;
+    format: string;
+    totalDuration: number;
+    chapterCount: number;
+  };
+  createdAt: number;
+  startedAt?: number;
+  completedAt?: number;
+}
+
+export interface JobProgress {
+  jobId: string;
+  progress: number;
+  currentStep: string;
+  sentencesProcessed?: number;
+  totalSentences?: number;
+}