perf: lazy load heavy dependencies and db drivers

Move database drivers and heavy server-side modules to be loaded
lazily via require() and dynamic import(). This reduces the initial
memory footprint and improves cold start performance for serverless
functions by avoiding loading unused dependencies on every request.

src/db/index.ts

@@ -1,14 +1,16 @@
-import { drizzle as drizzlePg } from 'drizzle-orm/node-postgres';
-import { drizzle as drizzleSqlite } from 'drizzle-orm/better-sqlite3';
 import { sql } from 'drizzle-orm';
-import { Pool } from 'pg';
-import Database from 'better-sqlite3';
 import path from 'path';
 import fs from 'fs';
 import * as schema from './schema';
 import * as authSchemaSqlite from './schema_auth_sqlite';
 import * as authSchemaPostgres from './schema_auth_postgres';
 
+// Database driver modules are loaded lazily via require() inside getDrizzleDB()
+// to avoid loading the unused driver (~15-20 MB each) in every serverless function.
+// require() is used instead of dynamic import() because getDrizzleDB() must remain
+// synchronous for the SQLite code path.
+
+
 const UNCLAIMED_USER_ID = 'unclaimed';
 
 // eslint-disable-next-line @typescript-eslint/no-explicit-any
@@ -62,12 +64,20 @@ function getDrizzleDB() {
   dbIsPostgres = !!process.env.POSTGRES_URL;
 
   if (dbIsPostgres) {
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    const { drizzle: drizzlePg } = require('drizzle-orm/node-postgres');
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    const { Pool } = require('pg');
     const pool = new Pool({
       connectionString: process.env.POSTGRES_URL,
     });
     dbInstance = drizzlePg(pool, { schema: { ...schema, ...authSchemaPostgres } });
   } else {
     // Fallback to SQLite
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    const { drizzle: drizzleSqlite } = require('drizzle-orm/better-sqlite3');
+    // eslint-disable-next-line @typescript-eslint/no-require-imports
+    const Database = require('better-sqlite3');
     const dbPath = path.join(process.cwd(), 'docstore', 'sqlite3.db');
     const dir = path.dirname(dbPath);
     if (!fs.existsSync(dir)) {

src/lib/server/auth.ts

@@ -5,17 +5,13 @@ import { drizzleAdapter } from "better-auth/adapters/drizzle";
 import { NextResponse } from 'next/server';
 import type { NextRequest } from 'next/server';
 import { db } from "@/db";
-import { rateLimiter } from "@/lib/server/rate-limiter";
 import { isAuthEnabled, isAnonymousAuthSessionsEnabled } from "@/lib/server/auth-config";
-import { deleteUserStorageData } from "@/lib/server/user-data-cleanup";
 import * as authSchemaSqlite from "@/db/schema_auth_sqlite";
 import * as authSchemaPostgres from "@/db/schema_auth_postgres";
-import {
+
-  transferUserAudiobooks,
+// Heavy modules (S3 SDK, blobstore, rate-limiter, claim-data) are loaded
-  transferUserDocuments,
+// lazily via dynamic import() inside the beforeDelete / onLinkAccount
-  transferUserPreferences,
+// callbacks to avoid inflating every serverless function that touches auth.
-  transferUserProgress,
-} from "@/lib/server/claim-data";
 
 // ...
 
@@ -80,6 +76,7 @@ const createAuth = () => betterAuth({
       enabled: true,
       beforeDelete: async (user) => {
         try {
+          const { deleteUserStorageData } = await import('@/lib/server/user-data-cleanup');
           await deleteUserStorageData(user.id, null);
         } catch (error) {
           console.error('[auth] Failed to clean up user storage before deletion:', error);
@@ -123,6 +120,12 @@ const createAuth = () => betterAuth({
                 newUserEmail: newUser.user.email,
               });
 
+              // Lazy-load heavy modules only when account linking actually happens
+              const [{ rateLimiter }, claimData] = await Promise.all([
+                import('@/lib/server/rate-limiter'),
+                import('@/lib/server/claim-data'),
+              ]);
+
               // Transfer rate limiting data (TTS char counts) from anonymous user to authenticated user
               try {
                 await rateLimiter.transferAnonymousUsage(anonymousUser.user.id, newUser.user.id);
@@ -134,7 +137,7 @@ const createAuth = () => betterAuth({
 
               // Transfer audiobooks from anonymous user to new authenticated user
               try {
-                const transferred = await transferUserAudiobooks(anonymousUser.user.id, newUser.user.id);
+                const transferred = await claimData.transferUserAudiobooks(anonymousUser.user.id, newUser.user.id);
                 if (transferred > 0) {
                   console.log(`Successfully transferred ${transferred} audiobook(s) from anonymous user ${anonymousUser.user.id} to user ${newUser.user.id}`);
                 }
@@ -145,7 +148,7 @@ const createAuth = () => betterAuth({
 
               // Transfer documents from anonymous user to new authenticated user
               try {
-                const transferred = await transferUserDocuments(anonymousUser.user.id, newUser.user.id);
+                const transferred = await claimData.transferUserDocuments(anonymousUser.user.id, newUser.user.id);
                 if (transferred > 0) {
                   console.log(`Successfully transferred ${transferred} document(s) from anonymous user ${anonymousUser.user.id} to user ${newUser.user.id}`);
                 }
@@ -156,7 +159,7 @@ const createAuth = () => betterAuth({
 
               // Transfer preferences from anonymous user to new authenticated user
               try {
-                const transferred = await transferUserPreferences(anonymousUser.user.id, newUser.user.id);
+                const transferred = await claimData.transferUserPreferences(anonymousUser.user.id, newUser.user.id);
                 if (transferred > 0) {
                   console.log(`Successfully transferred preferences from anonymous user ${anonymousUser.user.id} to user ${newUser.user.id}`);
                 }
@@ -167,7 +170,7 @@ const createAuth = () => betterAuth({
 
               // Transfer reading progress from anonymous user to new authenticated user
               try {
-                const transferred = await transferUserProgress(anonymousUser.user.id, newUser.user.id);
+                const transferred = await claimData.transferUserProgress(anonymousUser.user.id, newUser.user.id);
                 if (transferred > 0) {
                   console.log(`Successfully transferred ${transferred} progress row(s) from anonymous user ${anonymousUser.user.id} to user ${newUser.user.id}`);
                 }