Initial scaffold + canonical Activity model

Context
-------
New connector suite that syncs UK brokerage activity (Trading212,
InvestEngine, Schwab email-parsed, CSV drop-folder) into Wealthfolio.
Lives outside finance/ intentionally — finance/ is untouched per the
plan at ~/.claude/plans/let-s-work-on-linking-temporal-valiant.md.

This change
-----------
- Poetry project with httpx, typer, bs4, dev tools (pytest, mypy strict,
  ruff, yapf).
- Canonical Activity + Account models with the 6 UK tax wrappers
  (ISA/SIPP/GIA/LISA/JISA/WORKPLACE_PENSION) and the 12 Wealthfolio
  activity types from docs/activities/activity-types.md on the upstream.
- Validation invariants: BUY/SELL need qty+price, DIVIDEND/DEPOSIT/etc
  need amount — raises early so providers can't silently emit broken
  rows.
- to_wealthfolio_csv_row() shape matches Wealthfolio's CSV import;
  primary sink path per the plan.

Test plan
---------
## Automated
- poetry run pytest -q  →  7 passed in 0.03s
- poetry run mypy broker_sync tests  →  Success: no issues found in 4 source files
- poetry run ruff check .  →  All checks passed!
- poetry run yapf --diff --recursive broker_sync tests  →  no diff

## Manual Verification
Not applicable — pure data model, no runtime behaviour.

Closes: code-thw.1

tests/test_models.py

@@ -0,0 +1,126 @@
+from datetime import UTC, datetime
+from decimal import Decimal
+
+import pytest
+
+from broker_sync.models import (
+    Account,
+    AccountType,
+    Activity,
+    ActivityType,
+    FxRateSource,
+)
+
+
+def test_account_type_covers_uk_wrappers() -> None:
+    assert {
+        AccountType.ISA,
+        AccountType.SIPP,
+        AccountType.GIA,
+        AccountType.LISA,
+        AccountType.JISA,
+        AccountType.WORKPLACE_PENSION,
+    } <= set(AccountType)
+
+
+def test_activity_type_covers_wealthfolio_types() -> None:
+    # Mirrors Wealthfolio docs/activities/activity-types.md
+    for name in (
+            "BUY",
+            "SELL",
+            "DIVIDEND",
+            "INTEREST",
+            "DEPOSIT",
+            "WITHDRAWAL",
+            "TRANSFER_IN",
+            "TRANSFER_OUT",
+            "CONVERSION_IN",
+            "CONVERSION_OUT",
+            "FEE",
+            "TAX",
+    ):
+        assert hasattr(ActivityType, name), f"missing {name}"
+
+
+def test_activity_minimal_construct() -> None:
+    a = Activity(
+        external_id="t212:order:1",
+        account_id="t212-isa",
+        account_type=AccountType.ISA,
+        date=datetime(2026, 1, 15, tzinfo=UTC),
+        activity_type=ActivityType.BUY,
+        symbol="VUAG",
+        quantity=Decimal("10"),
+        unit_price=Decimal("85.5"),
+        currency="GBP",
+    )
+    assert a.external_id == "t212:order:1"
+    assert a.fee == Decimal("0")
+    assert a.fx_rate_source is None
+
+
+def test_buy_requires_qty_and_price() -> None:
+    with pytest.raises(ValueError, match="BUY.*quantity"):
+        Activity(
+            external_id="x",
+            account_id="a",
+            account_type=AccountType.GIA,
+            date=datetime(2026, 1, 1, tzinfo=UTC),
+            activity_type=ActivityType.BUY,
+            symbol="VUAG",
+            currency="GBP",
+        )
+
+
+def test_dividend_requires_amount() -> None:
+    with pytest.raises(ValueError, match="DIVIDEND.*amount"):
+        Activity(
+            external_id="x",
+            account_id="a",
+            account_type=AccountType.GIA,
+            date=datetime(2026, 1, 1, tzinfo=UTC),
+            activity_type=ActivityType.DIVIDEND,
+            currency="GBP",
+        )
+
+
+def test_wealthfolio_csv_row_shape() -> None:
+    a = Activity(
+        external_id="ie:2026-04:0001",
+        account_id="ie-isa",
+        account_type=AccountType.ISA,
+        date=datetime(2026, 4, 1, 10, 30, tzinfo=UTC),
+        activity_type=ActivityType.BUY,
+        symbol="VWRP",
+        quantity=Decimal("2.5"),
+        unit_price=Decimal("110.00"),
+        currency="GBP",
+        fee=Decimal("0.50"),
+        amount_gbp=Decimal("275.50"),
+        fx_rate_gbp=Decimal("1.0"),
+        fx_rate_source=FxRateSource.ECB_LIVE,
+        notes="sync:invest-engine:ie:2026-04:0001",
+    )
+    row = a.to_wealthfolio_csv_row()
+    # Wealthfolio activity import columns (from docs):
+    # date, activity_type, symbol, quantity, unit_price, currency, fee, amount, notes, ...
+    assert row["date"] == "2026-04-01T10:30:00+00:00"
+    assert row["activity_type"] == "BUY"
+    assert row["symbol"] == "VWRP"
+    assert row["quantity"] == "2.5"
+    assert row["unit_price"] == "110.00"
+    assert row["currency"] == "GBP"
+    assert row["fee"] == "0.50"
+    assert row["notes"] == "sync:invest-engine:ie:2026-04:0001"
+
+
+def test_account_construct() -> None:
+    acc = Account(
+        id="t212-isa",
+        name="Trading212 ISA",
+        account_type=AccountType.ISA,
+        currency="GBP",
+        provider="trading212",
+    )
+    assert acc.id == "t212-isa"
+    assert acc.account_type is AccountType.ISA