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>

payslip_ingest/processor.py

@@ -33,6 +33,17 @@ NON_PAYSLIP_TITLE_RE = re.compile(
     re.IGNORECASE,
 )
 
+# Some Paperless docs have no title at all — the title filter can't catch
+# them. These are detected by content signature in the pdftotext output.
+# Only apply to the first ~500 chars so we don't accidentally false-positive
+# a real payslip that happens to mention "P60" in a footnote somewhere.
+NON_PAYSLIP_CONTENT_RE = re.compile(
+    r"P60 End of Year Certificate"
+    r"|Employer's summary.+tax year ending"
+    r"|Take-home income per month",
+    re.IGNORECASE,
+)
+
 PDFTOTEXT_PATH = shutil.which("pdftotext")
 
 
@@ -71,8 +82,25 @@ async def process_document(
         return ProcessResult(doc_id=doc_id, status="skipped_non_payslip")
     pdf_bytes = await paperless.download_document(doc_id)
 
+    # Content-level non-payslip check (catches P60s with no Paperless title,
+    # personal income spreadsheets, etc.) before we burn extractor budget.
+    text_peek = _pdftotext(pdf_bytes) or ""
+    if NON_PAYSLIP_CONTENT_RE.search(text_peek[:500]):
+        log.info("skipping doc_id=%s — content matches non-payslip signature", doc_id)
+        return ProcessResult(doc_id=doc_id, status="skipped_non_payslip")
+
     extracted, which = await _extract(pdf_bytes, metadata, extractor)
 
+    # Sanity check: Viktor joined Meta UK in 2019. Any pay_date earlier
+    # than 2010 or a zero gross almost certainly means the extractor
+    # hallucinated on a non-payslip PDF that slipped past the title filter.
+    # Reject rather than poison the DB with a 1900-01-01 ghost row.
+    if extracted.pay_date.year < 2010:
+        raise ValueError(
+            f"doc_id={doc_id} extractor={which} produced implausible pay_date={extracted.pay_date}")
+    if extracted.gross_pay == 0 and extracted.net_pay == 0:
+        raise ValueError(f"doc_id={doc_id} extractor={which} produced zero gross and net")
+
     validated = validate_totals(extracted)
     if not validated:
         log.warning(