Add Meeting system subject and notification system

- Auto-create a locked "Meeting" subject for every user on registration
  and seed it for all existing users on startup
- Meeting subject cannot be deleted or renamed (is_system flag)
- 5-minute corner toast warning on Dashboard and TV with live countdown,
  dismiss button, and 1-minute re-notify if dismissed
- At start time: full-screen TV overlay with 30-second auto-dismiss,
  automatic pause of running block, switch to Meeting block, and
  auto-start of Meeting timer
- Web Audio API chimes: rising on warnings, falling at meeting start
- Update README with Meeting subject and notification system docs

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

backend/app/main.py

@@ -2,12 +2,15 @@ from contextlib import asynccontextmanager
 
 from fastapi import FastAPI, WebSocket, WebSocketDisconnect
 from fastapi.middleware.cors import CORSMiddleware
-from sqlalchemy import text
+from sqlalchemy import select, text
 from sqlalchemy.exc import OperationalError
+from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.config import get_settings
 from app.database import engine
 from app.models import Base
+from app.models.subject import Subject
+from app.models.user import User
 from app.routers import auth, users, children, subjects, schedules, sessions, logs, dashboard
 from app.routers import morning_routine, break_activity, admin
 from app.websocket.manager import manager
@@ -35,6 +38,27 @@ async def lifespan(app: FastAPI):
         await _add_column_if_missing(conn, "schedule_blocks", "break_time_minutes", "INT NULL")
         await _add_column_if_missing(conn, "children", "strikes", "INT NOT NULL DEFAULT 0")
         await _add_column_if_missing(conn, "users", "timezone", "VARCHAR(64) NOT NULL DEFAULT 'UTC'")
+        await _add_column_if_missing(conn, "subjects", "is_system", "TINYINT(1) NOT NULL DEFAULT 0")
+
+    # Seed Meeting system subject for any existing users who don't have one
+    from app.database import AsyncSessionLocal
+    async with AsyncSessionLocal() as db:
+        users_result = await db.execute(select(User).where(User.is_active == True))
+        all_users = users_result.scalars().all()
+        for user in all_users:
+            existing = await db.execute(
+                select(Subject).where(Subject.user_id == user.id, Subject.is_system == True)
+            )
+            if not existing.scalar_one_or_none():
+                db.add(Subject(
+                    user_id=user.id,
+                    name="Meeting",
+                    icon="📅",
+                    color="#6366f1",
+                    is_system=True,
+                ))
+        await db.commit()
+
     yield