Add Plex format coercion (.mp4 → .m4b)

Implement Plex-compatible file-extension coercion to avoid Plex silently ignoring .mp4 (and single-file .m4a) audiobooks (issue #166). Adds a DB migration and configuration key (plex_format_coercion_enabled, default true), exposes a toggle in the setup wizard and Admin Paths settings, and persists/reads the setting in the admin/setup APIs.

Introduces src/lib/utils/format-coercion.ts (coerceToPlexCompatible) and related constants in src/lib/constants/audio-formats.ts (PLEX_COMPATIBLE_EXTENSIONS, COERCION_RENAME_MAP, DRM_EXTENSIONS, TRANSCODE_REQUIRED_EXTENSIONS). The organize-files processor now runs coercion after organizing/tagging and before generating the filesHash and triggering scans; coercion is idempotent, never overwrites existing targets, logs warnings on DRM/transcode/permission errors, and is non-fatal.

Adds unit tests for the coercion util and updates processor & setup UI tests. Updates documentation (TABLEOFCONTENTS, file-organization, fixes/file-hash-matching, settings-pages) describing behavior, config, and constraints.

documentation/fixes/file-hash-matching.md

@@ -154,6 +154,12 @@ model Audiobook {
 - Hash generated AFTER merging
 - **Works correctly:** Hash reflects final organized state
 
+### Coerced Files (Plex Format Coercion)
+- Files renamed from `.mp4` → `.m4b` (or single-file `.m4a` → `.m4b`) by Plex format coercion
+- Hash generated AFTER coercion → reflects post-coercion filenames
+- **Works correctly going forward:** ABS sees post-coercion names, hash matches
+- **Pre-existing library entries** hashed before coercion was enabled will NOT match post-coercion files — retroactive library sweep is out of scope (see issue #166)
+
 ### Multiple Downloads (Same Book)
 - User re-downloads same audiobook (different edition/request)
 - Multiple records with same `filesHash`