fc3f: GET /api/artists/directory — cursor + q + platform filters with validation

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

backend/app/api/artists.py

@@ -1,11 +1,14 @@
 """FC-3a: plural /api/artists endpoints — POST (find-or-create) +
-autocomplete. The slug-routed singular /api/artist/<slug> blueprint
+autocomplete. FC-3f: GET /api/artists/directory (cursor-paginated
+browse grid). The slug-routed singular /api/artist/<slug> blueprint
 stays separate and unmodified."""
 
 from quart import Blueprint, jsonify, request
 
 from ..extensions import get_session
+from ..services.artist_directory_service import ArtistDirectoryService
 from ..services.artist_service import ArtistService
+from ..services.source_service import KNOWN_PLATFORMS
 
 artists_bp = Blueprint("artists", __name__, url_prefix="/api/artists")
 
@@ -42,3 +45,39 @@ async def autocomplete():
     return jsonify([
         {"id": a.id, "name": a.name, "slug": a.slug} for a in rows
     ])
+
+
+@artists_bp.route("/directory", methods=["GET"])
+async def directory():
+    """FC-3f: cursor-paginated artists directory.
+
+    Mirrors /api/tags/directory shape: { cards: [...], next_cursor }.
+    """
+    args = request.args
+
+    cursor = args.get("cursor") or None
+    q = args.get("q") or None
+    platform = args.get("platform") or None
+    limit_raw = args.get("limit", "60")
+
+    try:
+        limit = int(limit_raw)
+    except ValueError:
+        return jsonify({"error": "invalid_limit"}), 400
+    if limit < 1 or limit > 200:
+        return jsonify({"error": "invalid_limit"}), 400
+
+    if platform is not None and platform not in KNOWN_PLATFORMS:
+        return jsonify({"error": "unknown_platform"}), 400
+
+    async with get_session() as session:
+        svc = ArtistDirectoryService(session)
+        try:
+            page = await svc.list_artists(
+                q=q, platform=platform, cursor=cursor, limit=limit,
+            )
+        except ValueError:
+            # Service raises only on bad cursor (limit was validated above).
+            return jsonify({"error": "invalid_cursor"}), 400
+
+    return jsonify({"cards": page.cards, "next_cursor": page.next_cursor})