Add InvestEngine email parser — RFC 2822 v1/v2 line format

Context: The old finance/ app had a 324-line IE message parser with four
line-based variants (v1/v2/v3/v4) plus an HTML strategy and a CSV
fallback. Port into broker-sync so we can consume IE trade confirmation
emails as a backup to the live HTTP client (Phase 2b) while IE's public
API remains Bearer-only.

The upstream parser emits storage.model.Position; we emit canonical
Activity with the broker-sync invariants: account_id="invest-engine-primary"
(sink remaps to Wealthfolio UUID), account_type=ISA, currency=GBP, and
external_id="invest-engine:<fingerprint>" where the fingerprint is a
SHA-256 of (date|symbol|quantity|unit_price) — deterministic so repeat
imports of the same email dedup at the sync-record layer.

This change:
- Top-level `parse_invest_engine_email(raw_email: bytes) -> list[Activity]`
  extracts the text/plain body from an RFC 2822 message and dispatches to
  the line-based parser.
- `_parse_rfc2822_lines(body)` tries the v2 layout first (newer IE format
  where `Date: DD Month` is on line 2 and the year on line 3), then the
  v1 layout (where the day alone is on line 2 and `Month YYYY` on line 3).
  v3 and v4 variants are re-added in a follow-up if we find fixtures
  where they matter — initial fixture coverage hits v2.
- Drops the upstream `_ticker_post_processing` VUAG→VUAG.L hack.
  Wealthfolio's /import/check endpoint resolves exchange suffixes; the
  Trading212 provider also emits suffix-free tickers (e.g. `VUAG`), so
  staying consistent avoids double-mapping.
- Notes field records the parse-strategy tag ("rfc2822-v2") plus the
  matched line for debugging.

Test plan:
  poetry run pytest tests/providers/parsers/ -q
  → 3 passed in 0.03s
  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 the fixture email, call the parser, inspect the
returned Activity has symbol=VUAG, quantity=59.539562, unit_price=60.46,
date=2023-01-17, external_id starts with invest-engine:.

tests/fixtures/invest_engine/rfc2822_v2_single_buy.eml

@@ -0,0 +1,15 @@
+From: InvestEngine <no-reply@investengine.com>
+To: viktorbarzin@example.com
+Subject: Your portfolio has been updated
+Date: Tue, 17 Jan 2023 14:48:00 +0000
+MIME-Version: 1.0
+Content-Type: text/plain; charset=UTF-8
+Content-Transfer-Encoding: 8bit
+
+ <https://investengine.com/>          We've executed your orders and your
+portfolio has been updated           Client name: Redacted     Trading
+venue: London Stock Exchange     Type: Market Order(s)     Date: 17 January
+2023        Here's a summary of the trades we've made for you
+Vanguard S&P 500: VUAG     Bought 59.539562 @ £60.46 per share     Total:
+£3600.00     ISIN: IE00BFMXXD54, Order ID: 199510/2163746, Traded at
+2:48pm GMT/UTC            Take me to my updated portfolio

tests/providers/parsers/test_invest_engine.py

@@ -0,0 +1,44 @@
+from __future__ import annotations
+
+from datetime import datetime
+from decimal import Decimal
+from pathlib import Path
+
+from broker_sync.models import AccountType, ActivityType
+from broker_sync.providers.parsers.invest_engine import parse_invest_engine_email
+
+_FIXTURES = Path(__file__).parent.parent.parent / "fixtures" / "invest_engine"
+
+
+def _load(name: str) -> bytes:
+    return (_FIXTURES / name).read_bytes()
+
+
+# -- RFC 2822 body (v2-style, single BUY) --
+
+
+def test_rfc2822_single_buy_parses_to_one_activity() -> None:
+    activities = parse_invest_engine_email(_load("rfc2822_v2_single_buy.eml"))
+    assert len(activities) == 1
+    a = activities[0]
+    assert a.activity_type is ActivityType.BUY
+    assert a.symbol == "VUAG"
+    assert a.quantity == Decimal("59.539562")
+    assert a.unit_price == Decimal("60.46")
+    assert a.currency == "GBP"
+    assert a.date == datetime(2023, 1, 17)
+    assert a.account_id == "invest-engine-primary"
+    assert a.account_type is AccountType.ISA
+
+
+def test_rfc2822_external_id_is_deterministic() -> None:
+    a1 = parse_invest_engine_email(_load("rfc2822_v2_single_buy.eml"))[0]
+    a2 = parse_invest_engine_email(_load("rfc2822_v2_single_buy.eml"))[0]
+    assert a1.external_id == a2.external_id
+    assert a1.external_id.startswith("invest-engine:")
+
+
+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