Add per-block agenda overrides for daily sessions

- Add 📝 Agenda button to each block in Today's Schedule on the Dashboard
- Dialog allows setting a free-text activity/note for that block for the current day
- Agenda replaces subject options in the TV center panel while set; clears on session end
- Backend: new SessionBlockAgenda model, PUT /api/sessions/{id}/blocks/{block_id}/agenda
- Agendas included in dashboard snapshot and session_update WS broadcast
- New agenda_update WS event keeps TV in sync live when agenda is saved or cleared
- Update README with feature description, project structure, and WS event table

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/schemas/session.py

@@ -49,3 +49,4 @@ class DashboardSnapshot(BaseModel):
     is_break_active: bool = False    # whether break mode is currently active
     break_elapsed_seconds: int = 0   # seconds already elapsed in the break timer
     is_break_paused: bool = False    # whether the break timer is paused
+    block_agendas: dict[str, str] = {}  # block_id → agenda text override for today