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/library/[id]/route.ts

@@ -0,0 +1,44 @@
+import { NextRequest, NextResponse } from 'next/server'
+import { requireAdmin } from '@/lib/auth'
+import { getLibraryAiOverrides, setLibraryAiOverrides } from '@/lib/app-settings'
+
+export async function GET(
+  request: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const auth = await requireAdmin(request)
+  if (auth instanceof NextResponse) return auth
+
+  const { id } = await params
+  return NextResponse.json(getLibraryAiOverrides(id))
+}
+
+export async function PUT(
+  request: NextRequest,
+  { params }: { params: Promise<{ id: string }> }
+) {
+  const auth = await requireAdmin(request)
+  if (auth instanceof NextResponse) return auth
+
+  const { id } = await params
+
+  let body: Record<string, unknown>
+  try {
+    body = await request.json()
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 })
+  }
+
+  setLibraryAiOverrides(id, {
+    modelTagging: typeof body.modelTagging === 'string' ? body.modelTagging : undefined,
+    modelDescribe: typeof body.modelDescribe === 'string' ? body.modelDescribe : undefined,
+    modelExtract: typeof body.modelExtract === 'string' ? body.modelExtract : undefined,
+    modelTranslate: typeof body.modelTranslate === 'string' ? body.modelTranslate : undefined,
+    promptDescribe: typeof body.promptDescribe === 'string' ? body.promptDescribe : undefined,
+    promptTagger: typeof body.promptTagger === 'string' ? body.promptTagger : undefined,
+    promptExtract: typeof body.promptExtract === 'string' ? body.promptExtract : undefined,
+    promptTranslate: typeof body.promptTranslate === 'string' ? body.promptTranslate : undefined,
+  })
+
+  return NextResponse.json(getLibraryAiOverrides(id))
+}