fix(ingest): key formulas/figures on papers.id, not filename stem (audit C-10)

extract_formulas/extract_figures derive paper_id from Path(pdf_path).stem, which
need not equal papers.id (and won't, given canonical ids). Inserting that stem
violated the formulas/figures -> papers(id) FK on rich (.pdf) ingest. The DELETE
already used paper.id; the INSERTs now do too.

Strengthened the rich-ingest test: the fake formula/figure carry a filename-stem
paper_id distinct from papers.id, and the test asserts the inserted rows are
keyed on papers.id.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tests/ingest/test_ingest.py

@@ -351,9 +351,11 @@ def test_ingest_paper_rich_calls_formula_and_figure_parsers(tmp_path: Any) -> No
     mock_conn.executemany = MagicMock()
     mock_conn.commit = MagicMock()
 
-    fake_formula = FormulaChunk(paper_id=paper.id, page=1, raw_latex=r"\alpha", context="test")
+    # Extractors derive paper_id from the PDF filename stem ("paper"), which differs
+    # from papers.id — ingest must insert papers.id, not this stem (audit C-10).
+    fake_formula = FormulaChunk(paper_id="paper", page=1, raw_latex=r"\alpha", context="test")
     fake_figure = FigureChunk(
-        paper_id=paper.id, page=1, image_path="/tmp/fig.png", caption="Figure 1."
+        paper_id="paper", page=1, image_path="/tmp/fig.png", caption="Figure 1."
     )
 
     mock_settings = MagicMock()
@@ -380,6 +382,16 @@ def test_ingest_paper_rich_calls_formula_and_figure_parsers(tmp_path: Any) -> No
     assert result.formulas_upserted == 1
     assert result.figures_upserted == 1
 
+    # C-10: formula/figure rows must be keyed on papers.id, not the extractor's
+    # filename-stem paper_id, or the FK to papers(id) is violated.
+    cursor = mock_conn.cursor.return_value.__enter__.return_value
+    inserted_paper_ids = {
+        row[0] for call in cursor.executemany.call_args_list for row in call[0][1]
+    }
+    assert inserted_paper_ids == {paper.id}, (
+        f"formula/figure inserts must use papers.id, got {inserted_paper_ids}"
+    )
+
 
 def test_ingest_paper_no_rich_skips_parsers(tmp_path: Any) -> None:
     """When rich=False (default), formula and figure parsers are NOT called."""