Initial commit: event-driven UK payslip ingest service

Extracted from /home/wizard/code monorepo into its own repo so Woodpecker CI
can watch it. Identical content to /home/wizard/code commit e426028.

See README.md for overview, env vars, and Paperless workflow config.

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

tests/test_tax_year.py

@@ -0,0 +1,22 @@
+from datetime import date
+
+import pytest
+
+from payslip_ingest.tax_year import derive_tax_year
+
+
+@pytest.mark.parametrize(
+    ("pay_date", "expected"),
+    [
+        (date(2025, 4, 5), "2024/25"),
+        (date(2025, 4, 6), "2025/26"),
+        (date(2026, 4, 5), "2025/26"),
+        (date(2026, 4, 6), "2026/27"),
+        (date(2026, 12, 31), "2026/27"),
+        (date(2027, 1, 1), "2026/27"),
+        (date(2027, 4, 5), "2026/27"),
+        (date(2027, 4, 6), "2027/28"),
+    ],
+)
+def test_derive_tax_year(pay_date: date, expected: str) -> None:
+    assert derive_tax_year(pay_date) == expected