feat(F-16): chunk quality gate + section classification

- codex/quality.py: 3-signal filter (length / alpha-ratio / bib-score)
  + rule-based section classifier + run_quality_pass retroactive DB pass
- codex/ingest.py: promote quality imports to module level; apply
  filter_chunks before embedding; store section column in chunks INSERT
- codex/config.py: CHUNK_MIN_CHARS / CHUNK_MIN_ALPHA_RATIO / CHUNK_MAX_BIB_SCORE
- infra/schema.sql: ALTER TABLE chunks ADD COLUMN IF NOT EXISTS section TEXT
- .env.example: document F-16 quality thresholds
- codex/cli.py: quality run sub-command (scope by --paper-id or all papers)
- tests/quality/: 35 new tests covering all quality functions
- tests/ingest/: patch filter_chunks in source-path tests to isolate ingest

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/ingest/test_ingest.py

@@ -131,6 +131,7 @@ def test_ingest_paper_source_tex(tmp_path: Any) -> None:
         patch("codex.ingest.get_conn", side_effect=_make_conn_cm(mock_conn)),
         patch("codex.parsing.tex.latex_to_text", return_value="Hello world. Intro text."),
         patch("codex.parsing.tex.chunk_text", return_value=fake_chunks),
+        patch("codex.ingest.filter_chunks", side_effect=lambda chunks, settings: chunks),
     ):
         result = ingest_paper(paper.id, source_path=str(tex_file))
 
@@ -178,6 +179,7 @@ def test_ingest_paper_source_pdf(tmp_path: Any) -> None:
         patch("codex.parsing.nougat.pdf_to_markdown", return_value="pdf markdown text."),
         patch("codex.parsing.tex.chunk_text", return_value=fake_chunks),
         patch("codex.parsing.grobid.extract_references", return_value=grobid_refs),
+        patch("codex.ingest.filter_chunks", side_effect=lambda chunks, settings: chunks),
     ):
         result = ingest_paper(paper.id, source_path=str(pdf_file))