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>

tests/test_routes_dashboard.py

@@ -0,0 +1,18 @@
+"""Structural tests for the dashboard blueprint."""
+
+
+def test_dashboard_blueprint_registered():
+    from fabledassistant.routes.dashboard import dashboard_bp
+    assert dashboard_bp.name == "dashboard"
+    assert dashboard_bp.url_prefix == "/api/dashboard"
+
+
+def test_dashboard_blueprint_registered_in_app():
+    from fabledassistant.app import create_app
+    app = create_app()
+    assert "dashboard" in app.blueprints
+
+
+def test_dashboard_handler_callable():
+    from fabledassistant.routes import dashboard as dashboard_routes
+    assert callable(dashboard_routes.get_dashboard)