Add shareable search links and watched movie indicators

Share: "Copy Link" button generates a URL with ?s={history_id} that
loads the saved search results without auth. Recipients see the same
movie picks.

Watched: When viewing history results or shared searches, cards for
movies the logged-in user has since watched are dimmed with a green
"Watched" badge. Uses a new POST /api/library/watch-check endpoint.

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

app/routers/library.py

@@ -33,6 +33,28 @@ async def library_stats(request: Request):
         await db.close()
 
 
+@router.post("/watch-check")
+async def watch_check(request: Request):
+    """Given a list of jellyfin_ids, return which ones the current user has watched."""
+    user = await get_current_user(request)
+    body = await request.json()
+    ids = body.get("jellyfin_ids", [])
+    if not ids:
+        return {"watched": []}
+
+    db = await get_db()
+    try:
+        placeholders = ",".join("?" for _ in ids)
+        cursor = await db.execute(
+            f"SELECT jellyfin_id FROM watch_state WHERE user_id = ? AND is_played = 1 AND jellyfin_id IN ({placeholders})",
+            [user["id"]] + ids,
+        )
+        rows = await cursor.fetchall()
+        return {"watched": [row["jellyfin_id"] for row in rows]}
+    finally:
+        await db.close()
+
+
 @router.post("/sync")
 async def trigger_sync(request: Request):
     await get_current_user(request)