Add HTML table fallback for InvestEngine email parser

Context: Plain-text IE emails vanished around 2024-Q2 when IE switched to
an HTML-only template with per-order nested summary tables. The RFC 2822
line parser returns [] on those modern emails, so we need a fallback
that walks the HTML table structure.

Upstream _extract_from_html parsed a fixed DOM path (table[1].tr[10].
table) and only handled ONE order per email. The real IE HTML template
nests one summary <table> per ticker inside the second top-level table —
multiple orders in a single batched confirmation are common — so this
port walks every leaf table (no child <table>) and interprets each one
as an independent trade summary. Structural (non-leaf) tables are
skipped to avoid double-counting via get_text().

This change:
- `_parse_html_tables(body)` extracts the date once from the full text
  then walks leaf tables looking for "Bought N @ £P" rows.
- `_try_html_summary_table` parses one leaf; returns None on structural
  tables or missing ticker/qty/price — so a partial email yields only
  its intact orders (the "2 orders, 1 parseable → 1 returned" invariant
  works by construction without raising).
- `parse_invest_engine_email` now falls through text/plain → text/html
  in the multipart message, picking the first strategy that returns
  activities. Order matters: text/plain wins when both succeed because
  the RFC 2822 strategy is the more constrained grammar.
- Regexes are module-level constants so they compile once per process.

Fixture `html_two_orders.eml` is a minimal-but-realistic multipart email
with two nested summary tables (VUAG + SWDA), no personal data beyond
tickers/qty/price.

Test plan:
  poetry run pytest tests/providers/parsers/ -q
  → 5 passed in 0.16s
  poetry run mypy broker_sync/providers/parsers/invest_engine.py tests/providers/parsers/test_invest_engine.py
  → Success: no issues found in 2 source files
  poetry run ruff check broker_sync/providers/parsers/invest_engine.py tests/providers/parsers/test_invest_engine.py
  → All checks passed!
  poetry run yapf --diff broker_sync/providers/parsers/invest_engine.py tests/providers/parsers/test_invest_engine.py
  → clean (no diff)

Manual verification: load html_two_orders.eml, call parse_invest_engine_email,
assert len == 2 with both expected tickers (VUAG, SWDA) and numbers,
dates set to 2026-04-01.

tests/providers/parsers/test_invest_engine.py

@@ -42,3 +42,29 @@ def test_rfc2822_notes_record_parse_strategy() -> None:
     a = parse_invest_engine_email(_load("rfc2822_v2_single_buy.eml"))[0]
     assert a.notes is not None
     assert "rfc2822" in a.notes
+
+
+# -- HTML table body (multipart/alternative, two orders) --
+
+
+def test_html_body_parses_both_orders() -> None:
+    activities = parse_invest_engine_email(_load("html_two_orders.eml"))
+    assert len(activities) == 2
+    a, b = activities
+    assert a.symbol == "VUAG"
+    assert a.quantity == Decimal("10.5")
+    assert a.unit_price == Decimal("62.10")
+    assert a.date == datetime(2026, 4, 1)
+    assert a.account_id == "invest-engine-primary"
+    assert a.account_type is AccountType.ISA
+    assert a.activity_type is ActivityType.BUY
+    assert b.symbol == "SWDA"
+    assert b.quantity == Decimal("2.25")
+    assert b.unit_price == Decimal("85.40")
+    assert b.date == datetime(2026, 4, 1)
+
+
+def test_html_notes_record_html_strategy() -> None:
+    a = parse_invest_engine_email(_load("html_two_orders.eml"))[0]
+    assert a.notes is not None
+    assert "html" in a.notes