processor + parser: fix 3 backfill failure modes

## Context

After the first v2 backfill (commit f62c533), 72 of 73 real payslips
landed correctly, but three residual failure modes remained:

1. **doc_id=215** — a 1442-byte empty-text PDF that Claude
   hallucinated a `pay_date=1900-01-01 / gross=0 / net=0` row for.
   Data poison waiting to happen.
2. **doc_id=39** — a P60 End of Year Certificate. Got tagged
   `payslip` in Paperless, has no Paperless title, so the title-based
   filter couldn't catch it; the regex parser then happily pulled
   bogus numbers out of the P60 layout.
3. **doc_id=49** — a real June 2021 variant-A payslip with an
   `EE Discount BIK` line in BOTH Payments and Deductions at 12.00.
   The parser was configured to drop `EE Discount BIK` from
   `other_deductions` (treating it as a known mapped field), which
   caused validate_totals to fail by exactly 12.00.

## This change

### processor.py — defence in depth

- **`NON_PAYSLIP_CONTENT_RE`** — new regex run against the first
  500 chars of pdftotext output. Catches `P60 End of Year
  Certificate` and `Take-home income per month` (Viktor's comp
  estimation spreadsheet). First-500-char scoping keeps it from
  false-positiving a legit payslip that mentions "P60" in a
  footer.
- **Post-extraction sanity checks** — reject a ProcessResult if
  `pay_date.year < 2010` (Viktor joined Meta in 2019) or if
  `gross_pay == net_pay == 0`. These raise rather than insert,
  so the backfill's existing `except Exception` block logs and
  continues without poisoning the DB. Supersedes the 1900-01-01
  case that would otherwise slip through.

### meta_uk.py — variant A BIK fix

Removed `EE Discount BIK` from `VARIANT_A_DEDUCTIONS_KNOWN`. That
set filters items OUT of `other_deductions` (because they have
dedicated schema fields). `EE Discount BIK` has no dedicated
field — it should stay in `other_deductions` like Private Dental
and Private Medical so the validation math balances.

### Fixtures + tests

- New fixture `meta_uk_2021_06_variant_a_bik.txt` — real
  pdftotext from doc_id=49 — encodes the BIK-in-both-columns
  case so a regression would fail this fixture's validation test.
- `test_parses_variant_a_with_ee_discount_bik` — explicitly
  asserts `EE Discount BIK` lands in `other_deductions`.
- `test_rejects_implausible_pay_date`, `test_rejects_zero_gross_zero_net`
  — cover the two sanity-check branches.
- `test_skips_p60_by_content_when_title_is_null` — covers the
  content-based non-payslip filter.

## Test Plan

### Automated

```
$ poetry run pytest
============================== 57 passed in 2.42s ==============================
$ poetry run ruff check .
All checks passed!
$ poetry run mypy .
Success: no issues found in 24 source files
```

### Manual verification (after deploy + re-run backfill)

Expected DB shape:
- Total rows ≈ 71 (88 paperless tags − 15 non-payslip titles −
  2 null-title non-payslips caught by content filter)
- `validated = true` on ≥99% of rows
- No `pay_date < 2010` rows
- No rows with employer IS NULL

## Reproduce locally

1. `cd payslip-ingest && poetry run pytest`
2. Expected: 57 passed, including the 3 new processor tests and
   the 5 parametrised fixture-total-validation tests.

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

tests/test_meta_uk_parser.py

@@ -86,6 +86,31 @@ def test_parses_variant_c_2022_11() -> None:
     assert result.ytd_gross == Decimal("131034.64")
 
 
+def test_parses_variant_a_with_ee_discount_bik() -> None:
+    """June 2021 — variant A. Real pdftotext from doc_id=49.
+
+    Has an `EE Discount BIK` line present in BOTH Payments AND Deductions
+    blocks with value 12.00. Needs to land in `other_deductions` so the
+    validation formula accounts for it (earlier parser version filtered
+    it out, causing an off-by-12.00 validation failure).
+    """
+    result = parse_meta_uk(_load("meta_uk_2021_06_variant_a_bik.txt"))
+
+    assert result.pay_date == date(2021, 6, 30)
+    assert result.salary == Decimal("5096.65")
+    assert result.pension_sacrifice == Decimal("152.90")
+    assert result.rsu_vest == Decimal("0")  # RSU lines are YTD-only this period
+    assert result.rsu_offset == Decimal("0")
+    assert result.gross_pay == Decimal("5095.86")
+    assert result.income_tax == Decimal("1410.07")
+    assert result.national_insurance == Decimal("423.17")
+    assert result.student_loan == Decimal("244.00")
+    assert result.net_pay == Decimal("2906.51")
+    assert result.other_deductions.get("Private Dental Insurance") == Decimal("15.61")
+    assert result.other_deductions.get("Private Medical Insurance") == Decimal("84.50")
+    assert result.other_deductions.get("EE Discount BIK") == Decimal("12.00")
+
+
 def test_parses_variant_a_2021_08() -> None:
     """Aug 2021 — variant A. Real pdftotext from doc_id=43.
 
@@ -135,6 +160,7 @@ def test_raises_on_empty_text() -> None:
     "meta_uk_2025_03.txt",
     "meta_uk_2022_11_variant_c.txt",
     "meta_uk_2021_08_variant_a.txt",
+    "meta_uk_2021_06_variant_a_bik.txt",
 ])
 def test_all_fixtures_validate_totals(fixture_name: str) -> None:
     """Every fixture must satisfy gross - deductions ≈ net within 2p."""