Add cover URL edit button and modal

Rows without a cover (cover_url NULL or N/A) show a + button.
Clicking it opens a modal to enter a URL which is saved via new
PATCH /api/records/{id}/cover_url endpoint. cover_image is cleared
so n8n picks it up on the next backfill run.

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

api/main.py

@@ -544,6 +544,28 @@ async def get_records(
     }
 
 
+class SetCoverUrlBody(BaseModel):
+    cover_url: str
+
+
+@app.patch("/api/records/{record_id}/cover_url")
+async def set_record_cover_url(record_id: int, body: SetCoverUrlBody):
+    if pool is None:
+        raise HTTPException(503, detail="Datenbank nicht verbunden")
+    url = body.cover_url.strip()
+    if not url:
+        raise HTTPException(422, detail="cover_url darf nicht leer sein")
+    async with pool.acquire() as conn:
+        row = await conn.fetchrow(
+            f"UPDATE {qualified_table} SET cover_url = $1, cover_image = NULL, cover_mime_type = NULL WHERE id = $2 RETURNING id, cover_url",
+            url,
+            record_id,
+        )
+    if row is None:
+        raise HTTPException(404, detail=f"Eintrag mit ID {record_id} nicht gefunden")
+    return {"id": row["id"], "cover_url": row["cover_url"]}
+
+
 @app.patch("/api/records/{record_id}/seen")
 async def set_record_seen(record_id: int, body: SetSeenBody):
     """Einzelnen Eintrag als gesehen (seen=true) oder ungesehen (seen=false) markieren."""