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.

tests/app/setup/steps/PathsStep.test.tsx

@@ -23,6 +23,7 @@ const PathsHarness = ({
     downloadDir: '/downloads',
     mediaDir: '/media/audiobooks',
     metadataTaggingEnabled: true,
+    plexFormatCoercionEnabled: true,
     chapterMergingEnabled: false,
     ...initialState,
   });
@@ -84,7 +85,11 @@ describe('PathsStep', () => {
       <PathsHarness
         onNext={vi.fn()}
         onBack={vi.fn()}
-        initialState={{ metadataTaggingEnabled: false, chapterMergingEnabled: false }}
+        initialState={{
+          metadataTaggingEnabled: false,
+          plexFormatCoercionEnabled: false,
+          chapterMergingEnabled: false,
+        }}
       />
     );
 
@@ -97,4 +102,18 @@ describe('PathsStep', () => {
     expect(metadataToggle).toBeChecked();
     expect(chapterToggle).toBeChecked();
   });
+
+  it('renders plex format coercion toggle with default checked and toggles state', async () => {
+    render(<PathsHarness onNext={vi.fn()} onBack={vi.fn()} />);
+
+    const coercionToggle = screen.getByLabelText('Coerce file formats for Plex compatibility');
+
+    expect(coercionToggle).toBeChecked();
+
+    fireEvent.click(coercionToggle);
+    expect(coercionToggle).not.toBeChecked();
+
+    fireEvent.click(coercionToggle);
+    expect(coercionToggle).toBeChecked();
+  });
 });