Add AI home cam baby-watching state API (WATCHING / NOTWATCHING)

[Copilot]

Adds a singleton state resource to track whether the AI home cam is actively observing the baby, with JWT-authenticated GET and PUT endpoints.

New files

• Entity/CamStateType — enum with WATCHING / NOTWATCHING values
• Entity/CamState — JPA entity; singleton row (id = 1), stores @Enumerated(EnumType.STRING) state + updatedAt
• Repository/CamStateRepository — standard JpaRepository<CamState, Long>
• DTO/CamStateDTOs — UpdateRequest (@NotNull CamStateType state) and StateResponse (state, updatedAt)
• Service/CamStateService — upserts the singleton row; getState() returns NOTWATCHING / null as logical default when no row exists (no write on read)
• controller/CamStateController — GET /api/cam/state and PUT /api/cam/state

Auth

Both endpoints fall under the existing .anyRequest().authenticated() catch-all in SecurityConfig — no config changes needed. Unauthenticated requests get the standard 401 response.

API

GET /api/cam/state
Authorization: ******
→ 200 { "state": "NOTWATCHING", "updatedAt": null }

PUT /api/cam/state
Authorization: ******
{ "state": "WATCHING" }
→ 200 { "state": "WATCHING", "updatedAt": "2026-03-30T12:05:00.000Z" }

Invalid state value → 400 (Jackson enum deserialization failure)