add per-library AI model and prompt customization

- Add library_ai_settings table with migration for per-library overrides
- Extend AiConfig with editable prompt parts for description, tagging,
  extraction, and translation steps; defaults match previous hardcoded values
- Add getEffectiveAiConfig(libraryId) that merges global settings with
  library-level overrides (empty override falls through to global)
- Update all ai-tagger functions to use getEffectiveAiConfig and build
  prompts from configurable parts
- Add GET/PUT /api/ai-settings/library/[id] for per-library overrides
- Update /api/ai-settings GET/PUT to include prompt fields
- Add Prompts section and Library Overrides section to admin UI

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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

@@ -6,23 +6,40 @@ export async function GET(request: NextRequest) {
   const auth = await requireAdmin(request)
   if (auth instanceof NextResponse) return auth
 
-  const { endpoint, model, modelTagging, modelDescribe, modelExtract, modelTranslate, enabled } = getAiConfig()
+  const config = getAiConfig()
   const preferredLanguage = getPreferredLanguage()
-  return NextResponse.json({ endpoint, model, modelTagging, modelDescribe, modelExtract, modelTranslate, enabled, preferredLanguage })
+  return NextResponse.json({ ...config, preferredLanguage })
 }
 
 export async function PUT(request: NextRequest) {
   const auth = await requireAdmin(request)
   if (auth instanceof NextResponse) return auth
 
-  let body: { endpoint?: string; model?: string; modelTagging?: string; modelDescribe?: string; modelExtract?: string; modelTranslate?: string; enabled?: boolean; preferredLanguage?: string }
+  let body: {
+    endpoint?: string
+    model?: string
+    modelTagging?: string
+    modelDescribe?: string
+    modelExtract?: string
+    modelTranslate?: string
+    enabled?: boolean
+    preferredLanguage?: string
+    promptDescribe?: string
+    promptTagger?: string
+    promptExtract?: string
+    promptTranslate?: string
+  }
   try {
     body = await request.json()
   } catch {
     return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 })
   }
 
-  const { endpoint, model, enabled, preferredLanguage, modelTagging, modelDescribe, modelExtract, modelTranslate } = body
+  const {
+    endpoint, model, enabled, preferredLanguage,
+    modelTagging, modelDescribe, modelExtract, modelTranslate,
+    promptDescribe, promptTagger, promptExtract, promptTranslate,
+  } = body
 
   if (typeof endpoint !== 'string') {
     return NextResponse.json({ error: 'endpoint is required' }, { status: 400 })
@@ -42,6 +59,10 @@ export async function PUT(request: NextRequest) {
     typeof modelDescribe === 'string' ? modelDescribe : undefined,
     typeof modelExtract === 'string' ? modelExtract : undefined,
     typeof modelTranslate === 'string' ? modelTranslate : undefined,
+    typeof promptDescribe === 'string' ? promptDescribe : undefined,
+    typeof promptTagger === 'string' ? promptTagger : undefined,
+    typeof promptExtract === 'string' ? promptExtract : undefined,
+    typeof promptTranslate === 'string' ? promptTranslate : undefined,
   )
 
   if (typeof preferredLanguage === 'string' && preferredLanguage.trim()) {