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/app-settings.ts

@@ -36,3 +36,24 @@ export function updateScanConfig(schedule: string, enabled: boolean): void {
 export function setScanLastRan(ts: number): void {
   setSetting('scan_last_ran', String(ts))
 }
+
+// ─── AI Settings ─────────────────────────────────────────────────────────────
+
+interface AiConfig {
+  endpoint: string
+  model: string
+  enabled: boolean
+}
+
+export function getAiConfig(): AiConfig {
+  const endpoint = getSetting('ai_endpoint') ?? ''
+  const model = getSetting('ai_model') ?? ''
+  const enabled = getSetting('ai_enabled') === 'true'
+  return { endpoint, model, enabled }
+}
+
+export function updateAiConfig(endpoint: string, model: string, enabled: boolean): void {
+  setSetting('ai_endpoint', endpoint)
+  setSetting('ai_model', model)
+  setSetting('ai_enabled', enabled ? 'true' : 'false')
+}