extractor: track rsu_vest + rsu_offset separately from cash pay

UK payslips for equity-comp employees report RSU vests as notional pay
for HMRC only. A paired same-magnitude deduction (Shares Retained /
Stock Tax Withholding / RSU Offset) nets it back out of cash. The UK
payslip's income_tax line shows tax on the grossed-up total, but the
actual RSU tax is handled by Schwab (US broker) via share sale. No
cash flows through UK payroll for RSU.

Previously the extractor folded RSU notional into gross_pay and
income_tax, which inflated the dashboard numbers — a payslip with
£25k RSU vest looked like 2x salary with 80% tax rate.

Changes:
- schema: add rsu_vest + rsu_offset fields (default 0).
- db + alembic 0002: add two new NUMERIC(12,2) columns with server
  default 0 (backward-compatible; existing rows get 0).
- validate_totals: include rsu_offset in deductions sum so the
  gross + rsu_vest inflation is properly netted out.
- extraction prompt: explicit rules for identifying RSU lines by the
  common Meta/Sage/Workday labels, and to NOT put them in
  other_deductions.

Dashboards in a follow-up commit: cash_gross = gross_pay - rsu_vest,
effective tax rate based on cash metrics.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/test_processor.py

@@ -22,6 +22,8 @@ def _sample_extraction() -> ExtractedPayslip:
         pension_employee=Decimal("250.00"),
         pension_employer=Decimal("150.00"),
         student_loan=Decimal("100.00"),
+        rsu_vest=Decimal("0.00"),
+        rsu_offset=Decimal("0.00"),
         other_deductions={"cycle_to_work": Decimal("50.00")},
         net_pay=Decimal("3450.00"),
     )