add per-library AI model and prompt customization

- Add library_ai_settings table with migration for per-library overrides
- Extend AiConfig with editable prompt parts for description, tagging,
  extraction, and translation steps; defaults match previous hardcoded values
- Add getEffectiveAiConfig(libraryId) that merges global settings with
  library-level overrides (empty override falls through to global)
- Update all ai-tagger functions to use getEffectiveAiConfig and build
  prompts from configurable parts
- Add GET/PUT /api/ai-settings/library/[id] for per-library overrides
- Update /api/ai-settings GET/PUT to include prompt fields
- Add Prompts section and Library Overrides section to admin UI

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/db.ts

@@ -104,6 +104,7 @@ function initDb(db: Database.Database): void {
   migrateMediaTagsToItemKey(db)
   migrateMediaItemsAiTagged(db)
   migrateMediaItemsAiFields(db)
+  migrateLibraryAiSettings(db)
   seedAppSettings(db)
 }
 
@@ -259,6 +260,22 @@ function migrateMediaItemsAiFields(db: Database.Database): void {
   }
 }
 
+function migrateLibraryAiSettings(db: Database.Database): void {
+  db.exec(`
+    CREATE TABLE IF NOT EXISTS library_ai_settings (
+      library_id       TEXT PRIMARY KEY REFERENCES libraries(id) ON DELETE CASCADE,
+      model_tagging    TEXT,
+      model_describe   TEXT,
+      model_extract    TEXT,
+      model_translate  TEXT,
+      prompt_describe  TEXT,
+      prompt_tagger    TEXT,
+      prompt_extract   TEXT,
+      prompt_translate TEXT
+    );
+  `)
+}
+
 function migrateLibrariesType(db: Database.Database): void {
   const row = db
     .prepare("SELECT sql FROM sqlite_master WHERE type='table' AND name='libraries'")