Add SyncRecordStore for authoritative dedup

Context
-------
Wealthfolio's activity `notes` field is user-editable via the UI, so
using it as the dedup key would let a single note-edit in Wealthfolio
cause the next sync to create a duplicate. Stress-testing the plan
flagged this as the top structural risk.

This change
-----------
- SQLite-backed store at `/data/broker_sync.db` in production; keyed on
  (provider, account, external_id) so each provider's id space is
  scoped to its own account.
- `INSERT OR IGNORE` makes record() idempotent — second call with the
  same key is a no-op and preserves the original wealthfolio_activity_id
  plus first_seen timestamp.
- `filter_new()` is the integration point: provider fetches activities,
  hands them to the store, gets back only the unseen subset to submit
  to the Wealthfolio sink.
- Wealthfolio activity id returned by the API is persisted alongside
  each record so the HMRC FX reconciliation job can later PATCH the
  original activity rather than creating a new one.

Test plan
---------
## Automated
- poetry run pytest tests/test_dedup.py -v  →  6 passed
- poetry run mypy broker_sync tests  →  Success: no issues found in 6 source files
- poetry run ruff check .  →  All checks passed!

## Manual Verification
Not applicable for this layer — full end-to-end verification happens
once a provider + sink land (Phase 1 Trading212 and the auth spike).

broker_sync/models.py

@@ -78,9 +78,8 @@ class Activity:
     notes: str | None = None
 
     def __post_init__(self) -> None:
-        if self.activity_type in _QTY_PRICE_TYPES and (
-            self.quantity is None or self.unit_price is None
-        ):
+        if self.activity_type in _QTY_PRICE_TYPES and (self.quantity is None
+                                                       or self.unit_price is None):
             raise ValueError(f"{self.activity_type} requires quantity and unit_price")
         if self.activity_type in _AMOUNT_TYPES and self.amount is None:
             raise ValueError(f"{self.activity_type} requires amount")