feat(artist): move a source into another artist (#130 step 4)

Operator ask: a surface to merge new sources into existing artists (consolidate
the singleton artist a fresh add spins up). Enabled by the #130 slug decoupling —
the storage path is immutable, so re-attribution moves NO files. SourceService
.reassign moves the source, re-points its posts (Post.source_id==S) and the
images it contributed (ImageProvenance via S, scoped to the old artist so shared
images aren't stolen), and deletes the old artist if it's left fully empty (else
clears its subscription flag). POST /api/sources/<id>/reassign. Frontend: a
'Move…' action per source on the artist Management tab → artist-autocomplete
picker → confirm → routes to the target (whose slug is stable).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01CDgx8bQS5YrGRK76v8HUnM

backend/app/api/sources.py

@@ -136,6 +136,25 @@ async def delete_source(source_id: int):
     return "", 204
 
 
+@sources_bp.route("/<int:source_id>/reassign", methods=["POST"])
+async def reassign_source(source_id: int):
+    """Move this source (and the content it brought in) to another artist
+    (#130). Files don't move — the slug is immutable — so this just re-attributes
+    the source, its posts, and its images. Body: {target_artist_id}."""
+    body = await request.get_json(silent=True) or {}
+    target = body.get("target_artist_id")
+    if not isinstance(target, int):
+        return _bad("invalid_body", detail="target_artist_id (int) required")
+    async with get_session() as session:
+        try:
+            record = await SourceService(session).reassign(source_id, target)
+        except LookupError:
+            return _bad("not_found", status=404)
+        except ArtistNotFoundError:
+            return _bad("artist_not_found", detail="target artist not found", status=404)
+    return jsonify(record.to_dict())
+
+
 @sources_bp.route("/<int:source_id>/backfill", methods=["POST"])
 async def set_backfill(source_id: int):
     """Plan #693/#697 + #830: start/stop a backfill, or start a recovery /