ci: fix ruff E501 + mypy None-comparison warning

test_imap.py:49 — one-line comment ran past the 100-char line limit
introduced in commit c830856. Split the "£20,000 cap" note onto its
own line above the call.

test_fidelity_planviewer.py:108 — mypy flagged `offset.amount > 0`
where amount is typed Decimal | None. Added an explicit `is not None`
guard; runtime behaviour unchanged (we already check offset is not
None two lines earlier).

$ poetry run ruff check . → All checks passed!
$ poetry run mypy broker_sync tests → Success: no issues found in 43 source files
$ poetry run pytest -q → 133 passed, 1 skipped

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/providers/test_fidelity_planviewer.py

@@ -105,7 +105,7 @@ def test_gains_offset_emits_deposit_when_pot_exceeds_contributions() -> None:
     offset = _gains_offset_activity(holdings, txs, as_of)
     assert offset is not None
     assert offset.activity_type in (ActivityType.DEPOSIT, ActivityType.WITHDRAWAL)
-    assert offset.amount > 0
+    assert offset.amount is not None and offset.amount > 0
     assert offset.external_id == "fidelity:gains:2026-04-18"