feat: beta feedback widget — frictionless in-session feedback

Full-stack beta feedback system:

Backend:
- BetaFeedback model with reaction, category, text, page context
- POST /feedback/beta (any auth user), GET /feedback/beta (admin, filtered)
- Alembic migration 065 with indexes on user_id, reaction, created_at

Frontend:
- Persistent "Feedback" tab on right edge of all authenticated pages
- Slide-out panel: quick reaction (👍😐👎), category pills, optional text
- Auto-captures page URL and FlowPilot session ID
- Hidden on mobile (<640px), closes on Escape/outside click
- Shows "Thanks!" confirmation then auto-closes

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/api/router.py

@@ -29,6 +29,7 @@ from app.api.endpoints import public_templates
 from app.api.endpoints import admin_gallery
 from app.api.endpoints import uploads
 from app.api.endpoints import script_builder
+from app.api.endpoints import beta_feedback
 
 api_router = APIRouter()
 
@@ -83,3 +84,4 @@ api_router.include_router(public_templates.router)
 api_router.include_router(admin_gallery.router)
 api_router.include_router(uploads.router)
 api_router.include_router(script_builder.router)
+api_router.include_router(beta_feedback.router)