Add the core labeling data model (label sets, ad-hoc Main->Sub class
hierarchy, frames, bounding-box labels, per-frame/per-set label
status) behind a FastAPI app, with SQLite as the default swappable
DATABASE_URL. Multi-user support is attribution-only for now
(get-or-create by username, no login flow yet). Dataset
versioning/promotion is intentionally deferred - it needs its own
design pass around snapshot semantics.
Each test gets a fully isolated app+DB via create_app(database_url=...)
rather than relying on process-global state. 13/13 tests pass; also
verified live end-to-end against a running uvicorn instance.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
