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/components/ManageSubNav.tsx

@@ -8,6 +8,7 @@ const TABS = [
   { href: '/manage/tags', label: 'Tags' },
   { href: '/manage/users', label: 'Users' },
   { href: '/manage/scanning', label: 'Scanning' },
+  { href: '/manage/ai-tagging', label: 'AI Tagging' },
 ]
 
 export default function ManageSubNav() {