feat(state): add MongoBackend and URI-based backend factory

[himewel]

Summary

• Add MongoBackend in q2google/state/mongo.py storing sessions across three MongoDB collections (sessions, items, batches) that mirror the JsonFileBackend directory layout, enabling per-collection visibility into what is running
• Introduce build_backend(cfg) factory in q2google/state/__init__.py that selects the backend from a single Q2GOOGLE_STATE_URI env var — URI scheme routes to the right implementation; future backends need zero config changes
• pymongo is an optional extra (pip install q2google[mongo]); import q2google.state never fails without it — lazy import inside the factory raises a clear error only when a mongodb:// URI is actually requested

Changes

• q2google/state/mongo.py — new MongoBackend with load / save, lazy index creation, and session_id injected into item/batch documents for per-collection queries
• q2google/state/__init__.py — build_backend(cfg) factory + MongoBackend re-export
• q2google/config.py — state_uri: str | None field (Q2GOOGLE_STATE_URI); state_dir unchanged
• q2google/cli/_runner.py — accepts state_backend: SyncStateBackend instead of state_dir: Path
• q2google/cli/_app.py — uses build_backend(cfg); explicit --state-dir flag still overrides to filesystem
• pyproject.toml — [mongo] optional extra (pymongo>=4), mongomock>=4 in [dev]
• tests/conftest.py — parametrized backend fixture (json + mongo via mongomock); mongo variant auto-skips when dependencies are absent
• tests/test_state_backends.py — 5 protocol-compliance tests running for all backends (10 cases total)
• tests/test_sync.py — trimmed to filesystem-specific tests (legacy format, directory layout)
• docs/backends.md — new guide covering Filesystem, MongoDB, and Custom backends with collection schemas and install instructions
• docs/configuration.md, docs/getting-started.md, docs/ARCHITECTURE.md, docs/api/state.md, mkdocs.yml — updated to reference new backend

Test plan

• pip install q2google installs without pymongo — filesystem backend works as before
• pip install q2google[mongo] installs pymongo — Q2GOOGLE_STATE_URI=mongodb://... activates MongoBackend
• uv run pytest tests/ -vv — all 14 tests pass (12 backend-parametrized + 2 filesystem-specific)
• Setting an unknown URI scheme raises ValueError with a helpful message
• Setting mongodb:// URI without pymongo raises ImportError pointing to pip install q2google[mongo]

Made with Cursor