Context
-------
Trading212's `/equity/history/orders` is cursor-paginated via a
`nextPagePath` query-param in each response. Steady-state runs must
resume where the previous run finished, or we either miss fills (if
we start from 'now') or waste the 6/min rate limit walking history
we already imported (if we start from epoch).
A shared checkpoint store must live alongside the SyncRecordStore's
dedup DB on the /data PVC so CronJob pods can see progress from the
previous invocation. One file per (provider, account_id) because:
- T212 issues one API key per wrapper — ISA + Invest share no data.
- Plain JSON files are trivial to hand-edit during backfill if a
resume cursor gets stuck at a bad point.
This change
-----------
- broker_sync/providers/_checkpoint.py: `Checkpoint(dir, provider,
account_id)` with `load() -> str | None` and `save(cursor)`. Writes
`{cursor, updated_at}` to `<provider>-<account_id>.json`. Creates
parent directory lazily on first save so the PVC only needs a
mountpoint, not a pre-seeded layout.
- Provider-agnostic: no T212 knowledge. Will be reused for
InvestEngine in Phase 2.
- tests/providers/test_checkpoint.py: roundtrip, filename shape,
overwrite, per-account isolation, parent-dir creation, and a
malformed-file fallback (returns None rather than raising) so a
manual edit gone wrong does not brick the CronJob.
Test plan
---------
## Automated
- poetry run pytest -q → 48 passed in 0.47s
- poetry run mypy broker_sync tests → Success: no issues found in 24 source files
- poetry run ruff check . → All checks passed!
## Manual Verification
Not applicable — pure local-filesystem helper.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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