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/models/__init__.py

@@ -10,6 +10,7 @@ from app.models.morning_routine import MorningRoutineItem
 from app.models.break_activity import BreakActivityItem
 from app.models.strike import StrikeEvent
 from app.models.rule import RuleItem
+from app.models.session_block_agenda import SessionBlockAgenda
 
 __all__ = [
     "Base",
@@ -28,4 +29,5 @@ __all__ = [
     "BreakActivityItem",
     "StrikeEvent",
     "RuleItem",
+    "SessionBlockAgenda",
 ]