feat: align docs with code, finish_place, registered status, UI filters, tests, CI

- Add PLAN.md and sync backend docs, .env.example, API doc (404 details)
- Document mock DB and PORT/API_PORT in docs/backend.md; README monorepo + frontend/.env.example
- Migration 002: finish_place column, status registered; mapper and mock DB updated
- Frontend: registered status, finishPlace, calendar year/month filters, pace sparkline
- Extract createApp for tests; supertest + tsx; GitHub Actions CI

Made-with: Cursor

backend/migrations/002_finish_place_and_registered_status.sql

@@ -0,0 +1,5 @@
+ALTER TABLE races ADD COLUMN IF NOT EXISTS finish_place TEXT;
+
+ALTER TABLE races DROP CONSTRAINT IF EXISTS races_status_check;
+ALTER TABLE races ADD CONSTRAINT races_status_check
+  CHECK (status IS NULL OR status IN ('planned', 'registered', 'completed'));