Add Schwab email parser (port from finance/)

Schwab's workplace-RSU confirmation emails have 5 data td elements
with class='dark-background-body' align='right': date, direction, qty,
ticker, price-with-currency-sign. One email → one Activity.

- parse_schwab_email(raw_html) -> list[Activity] (1-item or empty)
- Empty on any parse failure (IMAP batch shouldn't crash on one bad mail)
- Deterministic external_id ('schwab📅ticker:type:qty') — stable
  across re-pulls so dedup works
- Hardcoded to account 'schwab-workplace' / AccountType.GIA / USD
- 6 unit tests: SELL + BUY happy path, malformed, missing cells,
  external-id stability, commas in price

Dropped from the original finance port:
- msg_timestamp-based external id (non-deterministic — would re-import
  on every IMAP walk). Replaced with a hash-stable key.
- Currency.from_sign() currency hack. Schwab US is USD-only; we'll add
  FX when that changes.

poetry run pytest -q   →  109 passed, 1 skipped
poetry run mypy        →  clean (added types-python-dateutil)
poetry run ruff check  →  clean

pyproject.toml

@@ -20,6 +20,7 @@ pytest-asyncio = "^0.23"
 mypy = "^1.11"
 ruff = "^0.6"
 yapf = "^0.43"
+types-python-dateutil = "^2.9.0.20260408"
 
 [tool.poetry.scripts]
 broker-sync = "broker_sync.cli:app"