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/test/route.ts

@@ -0,0 +1,47 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { requireAdmin } from '@/lib/auth'
+import { getAiConfig } from '@/lib/app-settings'
+
+export async function POST(request: NextRequest) {
+  const auth = await requireAdmin(request)
+  if (auth instanceof NextResponse) return auth
+
+  const { endpoint, model } = getAiConfig()
+
+  if (!endpoint) {
+    return NextResponse.json({ error: 'No endpoint configured' }, { status: 400 })
+  }
+
+  const url = endpoint.replace(/\/+$/, '') + '/chat/completions'
+
+  try {
+    const controller = new AbortController()
+    const timeout = setTimeout(() => controller.abort(), 10_000)
+
+    const res = await fetch(url, {
+      method: 'POST',
+      headers: { 'Content-Type': 'application/json' },
+      signal: controller.signal,
+      body: JSON.stringify({
+        model: model || 'test',
+        messages: [{ role: 'user', content: 'Hi' }],
+        max_tokens: 1,
+      }),
+    })
+
+    clearTimeout(timeout)
+
+    if (!res.ok) {
+      const text = await res.text().catch(() => '')
+      return NextResponse.json(
+        { error: `LLM returned ${res.status}: ${text.slice(0, 200)}` },
+        { status: 502 }
+      )
+    }
+
+    return NextResponse.json({ ok: true })
+  } catch (err) {
+    const message = err instanceof Error ? err.message : 'Unknown error'
+    return NextResponse.json({ error: `Connection failed: ${message}` }, { status: 502 })
+  }
+}