text extraction improvements: editable text and source language hint

- Extracted text in the tag panel is now an editable textarea; a Save
  button appears when the content is dirty and persists edits to the DB
- Source language input added next to Re-translate button; when filled,
  the translation prompt uses "translate from X to Y" for more accurate
  results
- New updateExtractedText() helper and PATCH /api/ai-tagging/fields
  endpoint to support saving edited text
- translateItemText/translateText accept optional sourceLanguage param

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

src/app/api/ai-tagging/fields/route.ts

@@ -1,6 +1,6 @@
 import { NextRequest, NextResponse } from 'next/server'
 import { requireLibraryAccess } from '@/lib/auth'
-import { getAiFields } from '@/lib/ai-tagger'
+import { getAiFields, updateExtractedText } from '@/lib/ai-tagger'
 
 export async function GET(request: NextRequest) {
   const { searchParams } = request.nextUrl
@@ -17,3 +17,27 @@ export async function GET(request: NextRequest) {
   const fields = getAiFields(itemKey)
   return NextResponse.json(fields)
 }
+
+export async function PATCH(request: NextRequest) {
+  let body: { itemKey?: string; extractedText?: string }
+  try {
+    body = await request.json()
+  } catch {
+    return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 })
+  }
+
+  const { itemKey, extractedText } = body
+  if (!itemKey || typeof itemKey !== 'string') {
+    return NextResponse.json({ error: 'itemKey is required' }, { status: 400 })
+  }
+  if (typeof extractedText !== 'string') {
+    return NextResponse.json({ error: 'extractedText is required' }, { status: 400 })
+  }
+
+  const libraryId = itemKey.split(':')[0]
+  const auth = await requireLibraryAccess(request, libraryId)
+  if (auth instanceof NextResponse) return auth
+
+  updateExtractedText(itemKey, extractedText)
+  return NextResponse.json({ ok: true })
+}

src/app/api/ai-tagging/translate/route.ts

@@ -3,14 +3,14 @@ import { requireLibraryAccess } from '@/lib/auth'
 import { translateItemText } from '@/lib/ai-tagger'
 
 export async function POST(request: NextRequest) {
-  let body: { itemKey?: string }
+  let body: { itemKey?: string; sourceLanguage?: string }
   try {
     body = await request.json()
   } catch {
     return NextResponse.json({ error: 'Invalid JSON body' }, { status: 400 })
   }
 
-  const { itemKey } = body
+  const { itemKey, sourceLanguage } = body
   if (!itemKey || typeof itemKey !== 'string') {
     return NextResponse.json({ error: 'itemKey is required' }, { status: 400 })
   }
@@ -20,7 +20,7 @@ export async function POST(request: NextRequest) {
   if (auth instanceof NextResponse) return auth
 
   try {
-    const translatedText = await translateItemText(itemKey)
+    const translatedText = await translateItemText(itemKey, sourceLanguage || undefined)
     return NextResponse.json({ translatedText })
   } catch (err) {
     const error = err as Error & { code?: string }