Context
-------
Closes out the Trading212 provider's retry + pagination surface so
the "Add Trading212Provider core fetch" commit has everything the
CronJob needs: cursor-based pagination, 429 honouring Retry-After,
jittered exponential backoff for 429-without-header and 5xx, bailout
after _MAX_RETRIES, and checkpoint-after-page semantics so a crashed
run resumes at the start of the unfinished page.
Also pins click<8.2 — typer 0.12 calls Parameter.make_metavar()
without a ctx argument, which click 8.2 removed; `broker-sync --help`
was crashing with TypeError until this pin. typer 0.15+ would also
fix it; the pin is lower friction.
One test fix: test_checkpoint_advances_only_after_page_yielded had a
handler that unconditionally returned a next_path → infinite loop. The
assertion was always about "a cursor was saved after page 1", so I
changed the handler to return page 2 as empty-with-no-next, which
terminates the loop cleanly.
Test plan
---------
## Automated
- poetry run pytest -q → 70 passed
- poetry run mypy broker_sync tests → Success: no issues found in 29 source files
- poetry run ruff check . → All checks passed!
- poetry run broker-sync --help → renders without crash; lists version + auth-spike
## Manual Verification
End-to-end against a live T212 key is in the next commit once the
CLI subcommand and pipeline land.
1eb3f78ea5