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/app/api/ai-settings/route.ts

@@ -0,0 +1,38 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { requireAdmin } from '@/lib/auth'
+import { getAiConfig, updateAiConfig } from '@/lib/app-settings'
+
+export async function GET(request: NextRequest) {
+  const auth = await requireAdmin(request)
+  if (auth instanceof NextResponse) return auth
+
+  const { endpoint, model, enabled } = getAiConfig()
+  return NextResponse.json({ endpoint, model, enabled })
+}
+
+export async function PUT(request: NextRequest) {
+  const auth = await requireAdmin(request)
+  if (auth instanceof NextResponse) return auth
+
+  let body: { endpoint?: string; model?: string; enabled?: boolean }
+  try {
+    body = await request.json()
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 })
+  }
+
+  const { endpoint, model, enabled } = body
+
+  if (typeof endpoint !== 'string') {
+    return NextResponse.json({ error: 'endpoint is required' }, { status: 400 })
+  }
+  if (typeof model !== 'string') {
+    return NextResponse.json({ error: 'model is required' }, { status: 400 })
+  }
+  if (typeof enabled !== 'boolean') {
+    return NextResponse.json({ error: 'enabled must be a boolean' }, { status: 400 })
+  }
+
+  updateAiConfig(endpoint, model, enabled)
+  return NextResponse.json({ endpoint, model, enabled })
+}