Add OpenAPI docs, frontend, migrations, and API updates

- OpenAPI: add missing endpoints (add-from-url, subscriptions, public availability)
- OpenAPI: CalendarSubscription schema, Subscriptions tag
- Frontend app
- Migrations: count_for_availability, subscriptions_sync, user_preferences, calendar_settings
- Config, rate limit, auth, calendar, booking, ICS, availability, user service updates

Made-with: Cursor

.env.example

@@ -1,15 +1,22 @@
 # Database
 DATABASE_URL=postgres://calendarapi:password@localhost:5432/calendarapi?sslmode=disable
 
-# Auth
+# Auth (REQUIRED: set secure value in production)
 JWT_SECRET=dev-secret-change-me
 
 # Redis (optional — enables background reminder jobs)
 # REDIS_ADDR=localhost:6379
 
 # Server
-SERVER_PORT=3019
+SERVER_PORT=8080
 ENV=development
 
 # Base URL (used for public iCal feed URLs; defaults to http://localhost:$SERVER_PORT)
 # BASE_URL=https://api.example.com
+
+# CORS (comma-separated origins; defaults to localhost:5173 for dev)
+# CORS_ORIGINS=https://app.example.com,https://www.example.com
+
+# Rate limiting
+# RATE_LIMIT_RPS=100
+# RATE_LIMIT_BURST=200