feat(pdf): extract statement header (account + period) + date format

Two related additions for the accountant workflow:

**1. Statement header extraction.** New
``extract_statement_metadata(pages)`` pulls the account number
and statement period out of the first page (falls back to
page 1+2 if either is missing on page 1 — Wells Fargo business
accounts put header info on page 2). Detected fields are
stamped onto EVERY transaction row so a multi-statement CSV is
self-attributing per row::

    {
      "date": "20250113",
      "description": "Coffee Shop",
      "amount_1": -4.50,
      "account_number": "****5678",
      "statement_period_start": "20250101",
      "statement_period_end": "20250131",
      ...
    }

Account-number regex is tolerant of masks (``****1234``),
hyphens (``1234-5678-9012``), and spaces. Period regex looks
for "Statement Period" / "From" / "Period Covered" labels plus
the first 1-2 full-year dates that follow. If only one date is
present near the label, it's used for both start and end (some
statements show only the closing date).

**2. Year inference for short dates.** When the row date is a
short ``01/13`` or ``Jan 13`` without a year, the scanner now
binds the year from the statement period's end date BEFORE
formatting. Doesn't handle the December-in-January-statement
cross-year case (rare; user can edit in the table).

**3. Configurable output date format.** New
``output_date_format`` parameter on ``scan_pdf_for_transactions``
defaults to ``%Y%m%d``. Applied to: the transaction date column
AND the statement period start/end fields. The page surfaces a
dropdown in Scan options with common presets (YYYYMMDD,
YYYY-MM-DD, MM/DD/YYYY, DD/MM/YYYY, ``Mon DD, YYYY``) plus a
Custom option that accepts a raw strftime string.

New helper: ``format_date(iso_str, fmt)`` converts ISO
``YYYY-MM-DD`` to any strftime; passes invalid input through
unchanged so the user can see what was actually there rather
than getting silent empties.

20 new tests cover: format_date, account-number extraction
(masked / hyphenated / spaced / no-label / short), period
extraction (standard / from-to / single-date / no-label),
metadata orchestrator (full header / no pages / page-2
fallback), year inference (US / dash / month-name / no-period /
unparseable), plus an end-to-end class that builds a header'd
PDF with short-date transactions and confirms metadata
attribution + year inference + format round-trip.

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

tests/test_pdf_extract.py

@@ -15,9 +15,14 @@ from __future__ import annotations
 from src.pdf_extract import (
     Page,
     WordBox,
+    _extract_account_number,
+    _extract_statement_period,
     _find_amount_tokens,
     _find_dates_in_words,
+    _infer_year_for_short_date,
     cluster_rows,
+    extract_statement_metadata,
+    format_date,
     parse_amount,
     parse_date,
 )
@@ -207,3 +212,134 @@ class TestFindAmountTokens:
 # test module — they need ``scan_pdf_for_transactions`` which in
 # turn uses ``extract_pages_auto``. The unit-test layer here pins
 # the building blocks; smoke tests pin the wiring.
+
+
+class TestFormatDate:
+    def test_yyyymmdd(self):
+        assert format_date("2026-01-13", "%Y%m%d") == "20260113"
+
+    def test_iso_passthrough(self):
+        assert format_date("2026-01-13", "%Y-%m-%d") == "2026-01-13"
+
+    def test_us(self):
+        assert format_date("2026-01-13", "%m/%d/%Y") == "01/13/2026"
+
+    def test_invalid_input_passes_through(self):
+        # Non-ISO input — return as-is so the user sees what was
+        # actually there rather than a silent empty string.
+        assert format_date("01/13", "%Y%m%d") == "01/13"
+
+    def test_none_or_empty(self):
+        assert format_date(None) == ""
+        assert format_date("") == ""
+
+
+class TestExtractAccountNumber:
+    def test_masked(self):
+        text = "Customer Name\nAccount Number: ****1234\nBalance"
+        assert _extract_account_number(text) == "****1234"
+
+    def test_with_hyphens(self):
+        text = "Account #: 1234-5678-9012"
+        assert _extract_account_number(text) == "1234-5678-9012"
+
+    def test_with_spaces(self):
+        text = "Account: 1234 5678 9012"
+        assert _extract_account_number(text) == "1234 5678 9012"
+
+    def test_no_label_no_match(self):
+        text = "Just some text with 1234567890 in it"
+        assert _extract_account_number(text) is None
+
+    def test_requires_at_least_four_digits(self):
+        # An "account" label followed by only XX shouldn't count.
+        text = "Account: XX"
+        assert _extract_account_number(text) is None
+
+
+class TestExtractStatementPeriod:
+    def test_standard_period(self):
+        text = "Statement Period: 01/01/2025 - 01/31/2025\nBalance"
+        start, end = _extract_statement_period(text)
+        assert start == "2025-01-01"
+        assert end == "2025-01-31"
+
+    def test_from_to(self):
+        text = "From 01/01/2025 to 01/31/2025"
+        start, end = _extract_statement_period(text)
+        assert start == "2025-01-01"
+        assert end == "2025-01-31"
+
+    def test_single_date_both_fields(self):
+        # When only one date appears near the label, return it for both.
+        text = "Statement Date: 01/31/2025"
+        start, end = _extract_statement_period(text)
+        assert start == "2025-01-31"
+        assert end == "2025-01-31"
+
+    def test_no_label_no_match(self):
+        text = "Some random text with 01/01/2025 in it"
+        start, end = _extract_statement_period(text)
+        # No "Period" / "From" / "Statement Date" label
+        assert (start, end) == (None, None)
+
+
+class TestExtractStatementMetadata:
+    def test_full_header(self):
+        pages = [Page(
+            page_no=1, width=600, height=800,
+            text=(
+                "ACME BANK\n"
+                "Customer: John Doe\n"
+                "Account Number: ****5678\n"
+                "Statement Period: 01/01/2025 - 01/31/2025\n"
+                "Beginning balance: $1,000.00\n"
+            ),
+            words=[],
+        )]
+        meta = extract_statement_metadata(pages)
+        assert meta["account_number"] == "****5678"
+        assert meta["period_start"] == "2025-01-01"
+        assert meta["period_end"] == "2025-01-31"
+
+    def test_no_pages(self):
+        meta = extract_statement_metadata([])
+        assert meta == {
+            "account_number": None,
+            "period_start": None,
+            "period_end": None,
+        }
+
+    def test_fallback_to_page_two(self):
+        # Page 1 has only account; period is on page 2.
+        p1 = Page(
+            page_no=1, width=600, height=800,
+            text="Account Number: ****1234\nBalance summary",
+            words=[],
+        )
+        p2 = Page(
+            page_no=2, width=600, height=800,
+            text="Statement Period: 02/01/2025 - 02/28/2025",
+            words=[],
+        )
+        meta = extract_statement_metadata([p1, p2])
+        assert meta["account_number"] == "****1234"
+        assert meta["period_start"] == "2025-02-01"
+        assert meta["period_end"] == "2025-02-28"
+
+
+class TestInferYearForShortDate:
+    def test_us_short_with_period_end(self):
+        assert _infer_year_for_short_date("01/13", "2025-01-31") == "2025-01-13"
+
+    def test_short_dash(self):
+        assert _infer_year_for_short_date("01-13", "2025-01-31") == "2025-01-13"
+
+    def test_month_name(self):
+        assert _infer_year_for_short_date("Jan 13", "2025-01-31") == "2025-01-13"
+
+    def test_no_period_end(self):
+        assert _infer_year_for_short_date("01/13", None) is None
+
+    def test_unparseable(self):
+        assert _infer_year_for_short_date("xx/yy", "2025-01-31") is None