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/models/__init__.py

@@ -49,6 +49,7 @@ from .notification import Notification
 from .psa_activity_log import PsaActivityLog
 from .file_upload import FileUpload
 from .ai_session_embedding import AISessionEmbedding
+from .beta_feedback import BetaFeedback
 
 __all__ = [
     "User",
@@ -112,4 +113,5 @@ __all__ = [
     "PsaActivityLog",
     "FileUpload",
     "AISessionEmbedding",
+    "BetaFeedback",
 ]