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>

alembic/versions/0002_add_rsu_columns.py

@@ -0,0 +1,33 @@
+"""Add rsu_vest and rsu_offset columns.
+
+UK payslips for Meta report RSU grants as notional pay (gross inflation)
+and offset them via a same-magnitude deduction. The cash gross Viktor
+cares about for dashboarding is gross_pay - rsu_vest. Track both for
+reporting + exactness; cash and tax-rate charts compute from them.
+"""
+import sqlalchemy as sa
+
+from alembic import op
+
+revision = "0002_add_rsu_columns"
+down_revision = "0001_initial"
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    op.add_column(
+        "payslip",
+        sa.Column("rsu_vest", sa.Numeric(12, 2), nullable=False, server_default=sa.text("0")),
+        schema="payslip_ingest",
+    )
+    op.add_column(
+        "payslip",
+        sa.Column("rsu_offset", sa.Numeric(12, 2), nullable=False, server_default=sa.text("0")),
+        schema="payslip_ingest",
+    )
+
+
+def downgrade() -> None:
+    op.drop_column("payslip", "rsu_offset", schema="payslip_ingest")
+    op.drop_column("payslip", "rsu_vest", schema="payslip_ingest")