fix(F-09): address review-gate findings — resource leak + idempotency

- mathpix.py + figures.py: wrap fitz page-loop in try/finally so
  doc.close() is guaranteed even on exception (no file-handle leak)
- ingest.py: DELETE FROM formulas/figures WHERE paper_id before re-insert
  so --rich re-ingest is idempotent (BIGSERIAL has no natural UNIQUE key;
  ON CONFLICT DO NOTHING was a no-op)

Gate: 158 passed, ruff clean, mypy clean

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

codex/ingest.py

@@ -220,13 +220,18 @@ def ingest_paper(
 
         if formulas or figures:
             with get_conn() as conn2:
+                with conn2.cursor() as cur:
+                    # Delete-before-insert keeps re-ingest idempotent (BIGSERIAL has no
+                    # natural UNIQUE key, so ON CONFLICT DO NOTHING never fires).
+                    cur.execute("DELETE FROM formulas WHERE paper_id = %s", (paper.id,))
+                    cur.execute("DELETE FROM figures  WHERE paper_id = %s", (paper.id,))
+
                 if formulas:
                     with conn2.cursor() as cur:
                         cur.executemany(
                             """
                             INSERT INTO formulas (paper_id, page, raw_latex, context, eq_label)
                             VALUES (%s, %s, %s, %s, %s)
-                            ON CONFLICT DO NOTHING
                             """,
                             [
                                 (f.paper_id, f.page, f.raw_latex, f.context, f.eq_label)
@@ -241,7 +246,6 @@ def ingest_paper(
                             """
                             INSERT INTO figures (paper_id, page, caption, image_path)
                             VALUES (%s, %s, %s, %s)
-                            ON CONFLICT DO NOTHING
                             """,
                             [
                                 (fig.paper_id, fig.page, fig.caption, fig.image_path)