parser + P60 ingest: split income_tax cash/RSU, add P60 ground-truth

Meta variant-B payslips gross up Taxable Pay for RSU and compute PAYE on
the grossed-up figure, so `income_tax` on the slip is the total PAYE
(cash + RSU-attributed). Dashboards that stacked the raw figure made
vest-month tax look ~2x higher than "cash tax paid". Introduce
`cash_income_tax = income_tax * (gross_pay - pension_sacrifice) /
taxable_pay` as a derived column alongside the raw figure. Dashboards
can now stack cash vs RSU-attributed tax as separate segments.

Also capture YTD column values of `RSU Tax Offset` and `RSU Excs Refund`
from the Payments grid — needed for reconciliation against HMRC annual
figures.

P60 ingest: new parser under `parsers/p60.py` anchoring on statutory
HMRC line labels (`Tax year to 5 April YYYY`, `Employer PAYE reference`,
`In this employment` pay/tax row, NI letter bands). Processor routes
documents carrying the `p60` Paperless tag to `_handle_p60` which
writes to the new `payslip_ingest.p60_reference` table (one row per
tax_year+employer). App lifespan resolves the tag id at startup; missing
tag disables dispatch without breaking payslip ingest. Paperless tag
creation + webhook config are manual follow-ups.

Migrations:
- 0004 — cash_income_tax + ytd_rsu_tax_offset + ytd_rsu_excs_refund on
  payslip, all nullable.
- 0005 — p60_reference table with (tax_year, employer) unique +
  paperless_doc_id unique for idempotent re-uploads.

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

tests/fixtures/meta_uk_p60_2024_25.txt

@@ -0,0 +1,51 @@
+P60 End of Year Certificate
+
+Tax year to 5 April 2025
+
+Employee's details
+
+Surname                           BARZIN
+First two forenames               VIKTOR
+National Insurance number         AA 12 34 56 A
+Works/Payroll number              254680
+
+Pay and Income Tax details
+
+                                  Pay                Tax deducted
+
+In previous employment(s)         £0.00              £0.00
+In this employment                £232,630.34        £95,820.11
+
+Total for year                    £232,630.34        £95,820.11
+
+Final tax code                    1257L
+
+National Insurance contributions in this employment
+
+NI table letter    Earnings at     Earnings above        Total of employee's
+                   LEL             LEL up to UEL         contributions in
+                                                         this employment
+A                  £6,396.00       £47,268.00            £5,172.40
+
+Statutory payments included in the pay 'In this employment' figure
+
+Statutory Maternity Pay            £0.00
+Statutory Paternity Pay            £0.00
+
+Student Loan deductions in this employment        £0.00
+
+Other details
+
+Your employer's full name and address
+
+Facebook UK Limited
+10 Brock Street
+London
+NW1 3FG
+
+Employer PAYE reference           120/FA12345
+
+This form shows your total pay for Income Tax purposes in this employment
+for the year. Any overtime, bonus, commission etc, Statutory Sick Pay,
+Statutory Maternity Pay, Statutory Paternity Pay or Shared Parental Pay,
+Statutory Parental Bereavement Pay is included.