feat(dashboard): GET /api/dashboard endpoint

Task 2 of #583. Minimal login-gated blueprint returning build_dashboard(uid);
registered in the app factory.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/fabledassistant/routes/dashboard.py

@@ -0,0 +1,13 @@
+"""Dashboard REST endpoint — the aggregated landing payload."""
+from quart import Blueprint, g, jsonify
+
+from fabledassistant.auth import login_required
+from fabledassistant.services.dashboard import build_dashboard
+
+dashboard_bp = Blueprint("dashboard", __name__, url_prefix="/api/dashboard")
+
+
+@dashboard_bp.get("")
+@login_required
+async def get_dashboard():
+    return jsonify(await build_dashboard(g.user.id))