Add AI-powered image tagging via local LLM

Adds automatic image tagging that runs as a post-scan phase, sending
thumbnails to an OpenAI-compatible vision API and applying matching
tags from the user-defined tag vocabulary.

- New ai-tagger module with batch processing, failure tolerance, and
  tag validation against existing vocabulary
- Admin settings page (Manage > AI Tagging) for endpoint, model, and
  enable toggle with connection testing
- DB migration for ai_tagged_at tracking column and AI config seeds
- Re-tag All support to queue items for reprocessing

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

src/lib/db.ts

@@ -102,6 +102,7 @@ function initDb(db: Database.Database): void {
   migrateMediaItemsSchema(db)
   migrateMediaItemsFingerprint(db)
   migrateMediaTagsToItemKey(db)
+  migrateMediaItemsAiTagged(db)
   seedAppSettings(db)
 }
 
@@ -110,6 +111,9 @@ function seedAppSettings(db: Database.Database): void {
     scan_schedule: '0 * * * *',
     scan_enabled: 'true',
     scan_last_ran: '',
+    ai_enabled: 'false',
+    ai_endpoint: '',
+    ai_model: '',
   }
   const insert = db.prepare(
     'INSERT OR IGNORE INTO app_settings (key, value) VALUES (?, ?)'
@@ -228,6 +232,15 @@ function migrateMediaTagsToItemKey(db: Database.Database): void {
   `)
 }
 
+function migrateMediaItemsAiTagged(db: Database.Database): void {
+  const row = db
+    .prepare("SELECT sql FROM sqlite_master WHERE type='table' AND name='media_items'")
+    .get() as { sql: string } | undefined
+  if (row && !row.sql.includes('ai_tagged_at')) {
+    db.exec('ALTER TABLE media_items ADD COLUMN ai_tagged_at INTEGER')
+  }
+}
+
 function migrateLibrariesType(db: Database.Database): void {
   const row = db
     .prepare("SELECT sql FROM sqlite_master WHERE type='table' AND name='libraries'")