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

broker_sync/models.py

@@ -0,0 +1,105 @@
+from __future__ import annotations
+
+from dataclasses import dataclass, field
+from datetime import datetime
+from decimal import Decimal
+from enum import StrEnum
+
+
+class AccountType(StrEnum):
+    ISA = "ISA"
+    SIPP = "SIPP"
+    GIA = "GIA"
+    LISA = "LISA"
+    JISA = "JISA"
+    WORKPLACE_PENSION = "WORKPLACE_PENSION"
+
+
+class ActivityType(StrEnum):
+    BUY = "BUY"
+    SELL = "SELL"
+    DIVIDEND = "DIVIDEND"
+    INTEREST = "INTEREST"
+    DEPOSIT = "DEPOSIT"
+    WITHDRAWAL = "WITHDRAWAL"
+    TRANSFER_IN = "TRANSFER_IN"
+    TRANSFER_OUT = "TRANSFER_OUT"
+    CONVERSION_IN = "CONVERSION_IN"
+    CONVERSION_OUT = "CONVERSION_OUT"
+    FEE = "FEE"
+    TAX = "TAX"
+
+
+class FxRateSource(StrEnum):
+    ECB_LIVE = "ECB_LIVE"
+    HMRC_MONTHLY = "HMRC_MONTHLY"
+
+
+_QTY_PRICE_TYPES = {ActivityType.BUY, ActivityType.SELL}
+_AMOUNT_TYPES = {
+    ActivityType.DIVIDEND,
+    ActivityType.INTEREST,
+    ActivityType.DEPOSIT,
+    ActivityType.WITHDRAWAL,
+    ActivityType.FEE,
+    ActivityType.TAX,
+    ActivityType.TRANSFER_IN,
+    ActivityType.TRANSFER_OUT,
+    ActivityType.CONVERSION_IN,
+    ActivityType.CONVERSION_OUT,
+}
+
+
+@dataclass
+class Account:
+    id: str
+    name: str
+    account_type: AccountType
+    currency: str
+    provider: str
+
+
+@dataclass
+class Activity:
+    external_id: str
+    account_id: str
+    account_type: AccountType
+    date: datetime
+    activity_type: ActivityType
+    currency: str
+    symbol: str | None = None
+    quantity: Decimal | None = None
+    unit_price: Decimal | None = None
+    amount: Decimal | None = None
+    fee: Decimal = field(default_factory=lambda: Decimal("0"))
+    amount_gbp: Decimal | None = None
+    fx_rate_gbp: Decimal | None = None
+    fx_rate_source: FxRateSource | None = None
+    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
+        ):
+            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")
+
+    def to_wealthfolio_csv_row(self) -> dict[str, str]:
+        return {
+            "date": self.date.isoformat(),
+            "activity_type": str(self.activity_type),
+            "symbol": self.symbol or "",
+            "quantity": _fmt(self.quantity),
+            "unit_price": _fmt(self.unit_price),
+            "currency": self.currency,
+            "fee": _fmt(self.fee),
+            "amount": _fmt(self.amount),
+            "notes": self.notes or "",
+        }
+
+
+def _fmt(v: Decimal | None) -> str:
+    if v is None:
+        return ""
+    return format(v, "f")