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/sessions.py

@@ -1,6 +1,7 @@
 from datetime import date
 
 from fastapi import APIRouter, Depends, HTTPException, status
+from pydantic import BaseModel
 from sqlalchemy.ext.asyncio import AsyncSession
 from sqlalchemy import select
 from sqlalchemy.orm import selectinload
@@ -12,6 +13,7 @@ from app.models.break_activity import BreakActivityItem
 from app.models.schedule import ScheduleBlock
 from app.models.subject import Subject  # noqa: F401 — needed for selectinload chain
 from app.models.session import DailySession, TimerEvent
+from app.models.session_block_agenda import SessionBlockAgenda
 from app.models.user import User
 from app.schemas.session import DailySessionOut, SessionStart, TimerAction
 from sqlalchemy import delete as sql_delete
@@ -85,6 +87,13 @@ async def _broadcast_session(db: AsyncSession, session: DailySession) -> None:
         )
         break_activities = [item.text for item in break_result.scalars().all()]
 
+    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()
+    }
+
     payload = {
         "event": "session_update",
         "session": {
@@ -98,6 +107,7 @@ async def _broadcast_session(db: AsyncSession, session: DailySession) -> None:
         "completed_block_ids": completed_ids,
         "morning_routine": morning_routine,
         "break_activities": break_activities,
+        "block_agendas": block_agendas,
     }
     await manager.broadcast(session.child_id, payload)
 
@@ -308,3 +318,49 @@ async def timer_action(
     await manager.broadcast(session.child_id, ws_payload)
 
     return session
+
+
+class AgendaUpdate(BaseModel):
+    text: str
+
+
+@router.put("/{session_id}/blocks/{block_id}/agenda", status_code=status.HTTP_204_NO_CONTENT)
+async def set_block_agenda(
+    session_id: int,
+    block_id: int,
+    body: AgendaUpdate,
+    current_user: User = Depends(get_current_user),
+    db: AsyncSession = Depends(get_db),
+):
+    result = await db.execute(
+        select(DailySession)
+        .join(Child)
+        .where(DailySession.id == session_id, Child.user_id == current_user.id)
+    )
+    session = result.scalar_one_or_none()
+    if not session:
+        raise HTTPException(status_code=404, detail="Session not found")
+
+    existing = await db.execute(
+        select(SessionBlockAgenda).where(
+            SessionBlockAgenda.session_id == session_id,
+            SessionBlockAgenda.block_id == block_id,
+        )
+    )
+    agenda = existing.scalar_one_or_none()
+
+    clean_text = body.text.strip()
+    if clean_text:
+        if agenda:
+            agenda.text = clean_text
+        else:
+            db.add(SessionBlockAgenda(session_id=session_id, block_id=block_id, text=clean_text))
+    elif agenda:
+        await db.delete(agenda)
+
+    await db.commit()
+    await manager.broadcast(session.child_id, {
+        "event": "agenda_update",
+        "block_id": block_id,
+        "text": clean_text,
+    })