Redesign TTS segment storage by decomposing the `tts_segments` table into two normalized tables: `tts_segment_entries` for unique segment identity and locator projection, and `tts_segment_variants` for per-settings audio variants. Update schema, queries, and API routes to use the new structure, including manifest pagination and segment resolution logic. Refactor segment locator handling to use stable projections and manifest cursors. Migrate tests and data cleanup utilities to support the new model.
BREAKING CHANGE: Database schema for TTS segments is now split into entry and variant tables; all code and consumers must use the new structure. Persisted data and APIs relying on the old `tts_segments` table are incompatible.
Remove legacy TTS segment manifest and audio endpoints in favor of new presigned and fallback audio URL approach. Update segment manifest structure to include `audioPresignUrl` and `audioFallbackUrl` fields, deprecating the previous `audioUrl`. Refactor segment ensure logic and TTS context to utilize the new playback source model. Add server utilities for generating presigned S3 URLs and introduce new API routes for presign and fallback audio retrieval. Clean up obsolete client API functions and types to reflect the new manifest format.
