Add Database Persistence

PROJECT CONTEXT:
My AI-built app holds data in component state or localStorage, so everything disappears on refresh or differs between devices.

GOAL:
Persist all meaningful app data in the database with real create/read/update/delete flows.

REQUIREMENTS:
- Inventory of every piece of state that should persist
- Database models and endpoints for each
- UI wired to load from and write to the backend
- Loading/empty/error states on every read
- Refresh-proof and multi-device-consistent data

CONSTRAINTS:
- localStorage is not the database for shared or important data
- Do not lose users' current in-memory data during migration if avoidable

DEFINITION OF DONE:
- Every important entity survives refresh
- Creates, edits, and deletes persist
- Lists load from the backend with proper states
- Two sessions see consistent data

COMMON FAILURES TO AVOID:
- Todo/items arrays living only in useState
- Saves that update state but never call an API
- Reads that never fetch, showing stale hardcoded data
- Missing loading states causing flash-of-empty bugs