schwab: detect vest-confirmation emails + emit VestEvent

Extends parse_schwab_email to handle Schwab's RSU Release Confirmation
emails alongside the existing trade confirmations. Adds:

- `VestEvent` dataclass in models.py — carries vest_date, ticker,
  shares_vested, shares_sold_to_cover, fmv_at_vest_usd, tax_withheld_usd.
  Written to payslip_ingest.rsu_vest_events by a postgres sink (pending
  a real email fixture + cross-service DB grant).
- `parse_schwab_email_full()` — new entry point returning both
  `list[Activity]` and `VestEvent | None`. The legacy
  `parse_schwab_email()` shape is preserved for existing callers.
- Vest-release dispatch heuristic: HTML body mentions "Release
  Confirmation" / "Award Vesting" / "RSU Release". On match, extract
  vest fields via label regexes; the full vest becomes a BUY Activity
  and the sell-to-cover slice becomes a SELL Activity at the same FMV
  (net zero cash on the day). Gross vest + sell-to-cover returned so
  Wealthfolio gets the full portfolio picture.
- Tests: 3 new (vest roundtrip, unparseable-vest safety, legacy shape
  preserved); existing 6 unchanged.

The regex heuristics will need tightening once a real email sample
exists — the HTML structure observed in public Schwab emails may
differ in material ways. For now, unmatched vest bodies return
empty-result (no Activity, no VestEvent) rather than crashing the
IMAP batch.

Part of: code-860

broker_sync/models.py

@@ -102,3 +102,27 @@ def _fmt(v: Decimal | None) -> str:
     if v is None:
         return ""
     return format(v, "f")
+
+
+@dataclass
+class VestEvent:
+    """Schwab RSU vest event — written to payslip_ingest.rsu_vest_events.
+
+    Carries both the gross vest (shares x FMV) and the sell-to-cover portion
+    (shares withheld for tax x FMV). Sibling Activity records (one BUY for
+    the full vest, one SELL for the sold-to-cover slice) are produced
+    separately for Wealthfolio.
+
+    USD-only at parse time; FX conversion happens at the postgres sink via
+    the ECB daily rate so the DB row carries both the raw USD figures and
+    the GBP-translated values for dashboard joins.
+    """
+    external_id: str  # schwab:{date}:{ticker}:VEST:{shares_vested}
+    vest_date: datetime
+    ticker: str
+    shares_vested: Decimal
+    shares_sold_to_cover: Decimal | None
+    fmv_at_vest_usd: Decimal
+    tax_withheld_usd: Decimal | None
+    source: str = "schwab_email"
+    raw: dict[str, str] = field(default_factory=dict)