Code quality: fix all ruff warnings, add CI/CD, improve test coverage

Sprint 13: comprehensive test fixtures and end-to-end hardening

Sprint 8: FastAPI routes — providers, jobs, exports, viewer

Sprint 7: persistence layer and job orchestration

Sprint 5: PAGE XML serializer and dual export

Sprint 3: vertical slice — PaddleOCR adapter, normalization pipeline, ALTO serializer

Sprint 0: project foundation — structure, settings, FastAPI skeleton