Add Provider protocol and normaliser

Context
-------
Every broker connector needs a uniform shape so the orchestrator can
fan out without knowing provider-specific details. Normalisation (GBP
conversion) lives outside providers on purpose — keeping providers
native-currency-emitters means we can re-normalise historical activity
when HMRC rates land without re-fetching from the broker.

This change
-----------
- providers/base.py: Provider Protocol with `accounts()` and async
  `fetch(since, before)` iterator. No abstract base class — duck-typed
  Protocol so each concrete provider stays independent.
- normaliser.py: takes a native Activity + FxCache, returns a copy
  with amount_gbp/fx_rate_gbp/fx_rate_source filled in. Two modes:
  qty*price for BUY/SELL, amount for DIVIDEND/DEPOSIT/etc.
- Namespace packages for providers/, providers/parsers/, sinks/ so
  future modules slot in cleanly.

Test plan
---------
## Automated
- poetry run pytest -q  →  23 passed
- poetry run mypy broker_sync tests  →  Success: no issues found in 14 source files
- poetry run ruff check .  →  All checks passed!

## Manual Verification
Not applicable at this layer.

broker_sync/normaliser.py

@@ -0,0 +1,35 @@
+from __future__ import annotations
+
+from dataclasses import replace
+from decimal import Decimal
+
+from broker_sync.fx import FxCache, convert_to_gbp
+from broker_sync.models import Activity, ActivityType
+
+_QTY_PRICE_TYPES = {ActivityType.BUY, ActivityType.SELL}
+
+
+def normalise_to_gbp(activity: Activity, *, cache: FxCache) -> Activity:
+    """Return a copy of `activity` with amount_gbp/fx_rate_gbp/fx_rate_source set.
+
+    Two cases:
+    - BUY/SELL: amount_gbp = quantity * unit_price * rate.
+    - Everything else (DIVIDEND/DEPOSIT/FEE/...): amount_gbp = amount * rate.
+
+    Source is always the cache's source tag (ECB_LIVE or HMRC_MONTHLY).
+    """
+    on_date = activity.date.date()
+    if activity.activity_type in _QTY_PRICE_TYPES:
+        assert activity.quantity is not None and activity.unit_price is not None
+        native_total: Decimal = activity.quantity * activity.unit_price
+    else:
+        assert activity.amount is not None
+        native_total = activity.amount
+
+    amount_gbp, rate, source = convert_to_gbp(native_total, activity.currency, on_date, cache=cache)
+    return replace(
+        activity,
+        amount_gbp=amount_gbp,
+        fx_rate_gbp=rate,
+        fx_rate_source=source,
+    )

broker_sync/providers/base.py

@@ -0,0 +1,32 @@
+from __future__ import annotations
+
+from collections.abc import AsyncIterator
+from datetime import datetime
+from typing import Protocol
+
+from broker_sync.models import Account, Activity
+
+
+class Provider(Protocol):
+    """Broker connector surface.
+
+    Each provider implementation is responsible for fetching raw broker
+    data, turning it into canonical `Activity` rows (with `account_id`
+    matching an `Account.id` from `accounts()`), and yielding them.
+
+    GBP conversion is performed by the shared normaliser, not here —
+    providers emit native currency and the caller converts.
+    """
+
+    name: str
+
+    def accounts(self) -> list[Account]:
+        ...
+
+    def fetch(
+        self,
+        *,
+        since: datetime | None = None,
+        before: datetime | None = None,
+    ) -> AsyncIterator[Activity]:
+        ...

tests/test_normaliser.py

@@ -0,0 +1,74 @@
+from datetime import UTC, date, datetime
+from decimal import Decimal
+from pathlib import Path
+
+import pytest
+
+from broker_sync.fx import FxCache
+from broker_sync.models import AccountType, Activity, ActivityType, FxRateSource
+from broker_sync.normaliser import normalise_to_gbp
+
+
+def _buy_usd(amount_usd: Decimal = Decimal("100")) -> Activity:
+    return Activity(
+        external_id="schwab:1",
+        account_id="schwab-rsu",
+        account_type=AccountType.GIA,
+        date=datetime(2026, 4, 1, tzinfo=UTC),
+        activity_type=ActivityType.BUY,
+        symbol="META",
+        quantity=Decimal("1"),
+        unit_price=amount_usd,
+        currency="USD",
+        fee=Decimal("0"),
+    )
+
+
+def test_gbp_activity_unchanged(tmp_path: Path) -> None:
+    cache = FxCache(tmp_path / "fx.db")
+    a = Activity(
+        external_id="t212:1",
+        account_id="t212-isa",
+        account_type=AccountType.ISA,
+        date=datetime(2026, 4, 1, tzinfo=UTC),
+        activity_type=ActivityType.BUY,
+        symbol="VUAG",
+        quantity=Decimal("1"),
+        unit_price=Decimal("100"),
+        currency="GBP",
+    )
+    out = normalise_to_gbp(a, cache=cache)
+    assert out.amount_gbp == Decimal("100")
+    assert out.fx_rate_gbp == Decimal("1")
+    assert out.fx_rate_source is FxRateSource.ECB_LIVE
+
+
+def test_usd_buy_converts_using_cache(tmp_path: Path) -> None:
+    cache = FxCache(tmp_path / "fx.db")
+    cache.put("USD", date(2026, 4, 1), Decimal("0.80"), FxRateSource.ECB_LIVE)
+    out = normalise_to_gbp(_buy_usd(Decimal("100")), cache=cache)
+    assert out.amount_gbp == Decimal("80.00")
+    assert out.fx_rate_gbp == Decimal("0.80")
+    assert out.fx_rate_source is FxRateSource.ECB_LIVE
+
+
+def test_dividend_amount_drives_gbp(tmp_path: Path) -> None:
+    cache = FxCache(tmp_path / "fx.db")
+    cache.put("USD", date(2026, 4, 1), Decimal("0.80"), FxRateSource.ECB_LIVE)
+    div = Activity(
+        external_id="schwab:div:1",
+        account_id="schwab-rsu",
+        account_type=AccountType.GIA,
+        date=datetime(2026, 4, 1, tzinfo=UTC),
+        activity_type=ActivityType.DIVIDEND,
+        currency="USD",
+        amount=Decimal("50"),
+    )
+    out = normalise_to_gbp(div, cache=cache)
+    assert out.amount_gbp == Decimal("40.00")
+
+
+def test_missing_rate_raises(tmp_path: Path) -> None:
+    cache = FxCache(tmp_path / "fx.db")
+    with pytest.raises(LookupError):
+        normalise_to_gbp(_buy_usd(), cache=cache)