feat(fc3c): POST /api/sources/<id>/check — enqueue download

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

backend/app/api/sources.py

@@ -1,8 +1,10 @@
-"""FC-3a: CRUD over Source rows."""
+"""FC-3a: CRUD over Source rows. FC-3c adds POST /<id>/check."""
 
 from quart import Blueprint, jsonify, request
+from sqlalchemy import select
 
 from ..extensions import get_session
+from ..models import DownloadEvent, Source
 from ..services.source_service import (
     KNOWN_PLATFORMS,
     ArtistNotFoundError,
@@ -114,3 +116,41 @@ async def delete_source(source_id: int):
         except LookupError:
             return _bad("not_found", status=404)
     return "", 204
+
+
+@sources_bp.route("/<int:source_id>/check", methods=["POST"])
+async def check_source(source_id: int):
+    """FC-3c: enqueue a download for this source.
+
+    Returns 202 with the new DownloadEvent id. If a pending/running
+    event already exists for this source, returns 409 with that id."""
+    async with get_session() as session:
+        source = (await session.execute(
+            select(Source).where(Source.id == source_id)
+        )).scalar_one_or_none()
+        if source is None:
+            return _bad("not_found", status=404, detail=f"source id={source_id}")
+        if not source.enabled:
+            return _bad("source_disabled", detail="enable the source first")
+
+        in_flight = (await session.execute(
+            select(DownloadEvent.id).where(
+                DownloadEvent.source_id == source_id,
+                DownloadEvent.status.in_(["pending", "running"]),
+            ).order_by(DownloadEvent.id.desc()).limit(1)
+        )).scalar_one_or_none()
+        if in_flight is not None:
+            return jsonify(
+                {"download_event_id": in_flight, "status": "already_running"}
+            ), 409
+
+        event = DownloadEvent(source_id=source_id, status="pending")
+        session.add(event)
+        await session.commit()
+        await session.refresh(event)
+        event_id = event.id
+
+    from ..tasks.download import download_source
+    download_source.delay(source_id)
+
+    return jsonify({"download_event_id": event_id, "status": "pending"}), 202