Moves business logic from API handlers into internal/service/ so that
both the REST API and the upcoming dashboard can share the same operations
without duplicating code. API handlers now delegate to the service layer
and only handle HTTP-specific concerns (request parsing, response formatting).
Implement email/password auth with JWT sessions and API key auth for
sandbox lifecycle. Users get a default team on signup; sandboxes,
snapshots, and API keys are scoped to teams.
- Add user, team, users_teams, and team_api_keys tables (goose migrations)
- Add JWT middleware (Bearer token) for user management endpoints
- Add API key middleware (X-API-Key header, SHA-256 hashed) for sandbox ops
- Add signup/login handlers with transactional user+team creation
- Add API key CRUD endpoints (create/list/delete)
- Replace owner_id with team_id on sandboxes and templates
- Update all handlers to use team-scoped queries
- Add godotenv for .env file loading
- Update OpenAPI spec and test UI with auth flows
