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/scanner.ts

@@ -10,6 +10,7 @@ import { scanGamesLibrary } from './games'
 import { getThumbnailPath } from './thumbnails'
 import { computeFingerprint } from './fingerprint'
 import { reKeyMediaItem } from './tags'
+import { runAiTagging } from './ai-tagger'
 
 let scanRunning = false
 
@@ -70,6 +71,10 @@ export async function runLibraryScan(library: Library): Promise<void> {
       await scanMixed(library, libraryRoot)
       break
   }
+
+  await runAiTagging(library, libraryRoot).catch((err) =>
+    console.error(`[ai-tagger] Error tagging library "${library.name}":`, err)
+  )
 }
 
 // ---------------------------------------------------------------------------