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/routers/dashboard.py

@@ -14,6 +14,7 @@ from app.models.child import Child
 from app.models.morning_routine import MorningRoutineItem
 from app.models.break_activity import BreakActivityItem
 from app.models.schedule import ScheduleBlock
+from app.models.session_block_agenda import SessionBlockAgenda
 from app.models.subject import Subject  # noqa: F401 — needed for selectinload chain
 from app.models.session import DailySession, TimerEvent
 from app.schemas.session import DashboardSnapshot
@@ -109,6 +110,15 @@ async def get_dashboard(tv_token: int, db: AsyncSession = Depends(get_db)):
     )
     break_activities = [item.text for item in break_result.scalars().all()]
 
+    block_agendas: dict[str, str] = {}
+    if session:
+        agendas_result = await db.execute(
+            select(SessionBlockAgenda).where(SessionBlockAgenda.session_id == session.id)
+        )
+        block_agendas = {
+            str(item.block_id): item.text for item in agendas_result.scalars().all()
+        }
+
     return DashboardSnapshot(
         session=session,
         child=child,
@@ -121,4 +131,5 @@ async def get_dashboard(tv_token: int, db: AsyncSession = Depends(get_db)):
         is_break_active=is_break_active,
         break_elapsed_seconds=break_elapsed_seconds,
         is_break_paused=is_break_paused,
+        block_agendas=block_agendas,
     )