fix(database): read the verify streams one row at a time

The provenance and review-evidence checks iterated their cursors; on a
managed connection iteration prefetches 128 rows, so up to 128 large values
were held at once while the docstrings promised one. Both now use fetchone.

database/models.py

@@ -554,7 +554,10 @@ class PromptModel:
             "WHERE pp.positive_prompt_id IS NOT gi.prompt_id LIMIT 1"
         ).fetchone():
             return "Image pair association has a different positive prompt owner"
-        for row in conn.execute("SELECT provenance FROM image_prompt_pairs"):
+        # fetchone, not iteration: a managed connection's cursor prefetches
+        # ITER_CHUNK rows when iterated, and each provenance may be ~4 MB.
+        cursor = conn.execute("SELECT provenance FROM image_prompt_pairs")
+        while (row := cursor.fetchone()) is not None:
             error = pair_provenance_error(row[0])
             if error:
                 return error

database/review_schema.py

@@ -380,15 +380,21 @@ def _structure_error(conn: sqlite3.Connection) -> Optional[str]:
 
 
 def _evidence_error(conn: sqlite3.Connection) -> Optional[str]:
-    """Stream every evidence document once; one row is held at a time."""
-    for evidence_hash, text in conn.execute(
+    """Stream every evidence document once; one row is held at a time.
+
+    Rows are read with ``fetchone``: iterating a managed connection's cursor
+    would prefetch ``ITER_CHUNK`` (128) rows (``database/managed_connection.py``).
+    """
+    items = conn.execute(
         f"SELECT evidence_hash, evidence FROM {ITEMS_TABLE}"  # nosec B608
-    ):
+    )
+    while (row := items.fetchone()) is not None:
+        evidence_hash, text = row[0], row[1]
         if not _is_canonical(text) or evidence_digest(text) != evidence_hash:
             return f"Review queue table {ITEMS_TABLE} contains invalid evidence"
-    for (text,) in conn.execute(
-        f"SELECT evidence FROM {DECISIONS_TABLE}"  # nosec B608
-    ):
+    decisions = conn.execute(f"SELECT evidence FROM {DECISIONS_TABLE}")  # nosec B608
+    while (row := decisions.fetchone()) is not None:
+        text = row[0]
         if not _is_canonical(text):
             return f"Review queue table {DECISIONS_TABLE} contains invalid evidence"
     return None