fix(snippets): backfill must also catch JSON null, not just SQL NULL

Integration lane caught it (run 2984): the backfill reported 0 rows to fill
and left `data` unset. `IS NULL` was the whole predicate, but a JSONB column
has two empty states. SQLAlchemy's JSON types default to
`none_as_null=False`, so assigning Python `None` persists the JSON encoding
of null — `IS NULL` walks straight past it.

Migration 0070 left genuine SQL NULLs, so the product path was right; the
test was constructing the wrong shape with `data=None`. Fixed both ways,
because both states mean "no usable mirror":
- predicate is now `data IS NULL OR jsonb_typeof(data) = 'null'`;
- the legacy-row test OMITS `data` (a real SQL NULL, 0070's actual shape),
  and a second test covers the JSON-null shape and asserts the premise with
  `jsonb_typeof` rather than assuming it.

Refs #2083.

src/scribe/services/snippets.py

@@ -32,7 +32,7 @@ import asyncio
 import logging
 import re
 
-from sqlalchemy import select
+from sqlalchemy import func, or_, select
 
 from scribe.models import async_session
 from scribe.models.note import Note
@@ -386,8 +386,16 @@ async def backfill_snippet_data(*, batch: int = 500) -> int:
     filled row is skipped forever after, and a snippet with no structured fields
     at all settles at `{}` rather than staying NULL and being re-scanned. Trashed
     rows are included so a later restore comes back queryable.
+
+    "Unfilled" means SQL NULL *or* JSON `null` — two different states in a JSONB
+    column, and only the first is what migration 0070 left behind. SQLAlchemy's
+    JSON types default to ``none_as_null=False``, so assigning Python ``None`` to
+    this column persists the JSON encoding of null rather than SQL NULL; an
+    ``IS NULL`` test alone walks straight past such a row and reports nothing to
+    do. Both mean "no usable mirror", so both are filled.
     """
     filled = 0
+    unfilled = or_(Note.data.is_(None), func.jsonb_typeof(Note.data) == "null")
     async with async_session() as session:
         while True:
             rows = list(
@@ -395,7 +403,7 @@ async def backfill_snippet_data(*, batch: int = 500) -> int:
                     await session.execute(
                         select(Note)
                         .where(Note.note_type == SNIPPET_NOTE_TYPE)
-                        .where(Note.data.is_(None))
+                        .where(unfilled)
                         .limit(batch)
                     )
                 )

tests/test_integration_snippet_locations.py

@@ -17,7 +17,7 @@ from unittest.mock import AsyncMock, patch
 
 import pytest
 import pytest_asyncio
-from sqlalchemy import delete, select
+from sqlalchemy import delete, func, select
 
 from scribe.models import async_session, engine
 from scribe.models.note import Note
@@ -241,7 +241,13 @@ async def test_sql_and_python_dialects_agree_on_the_same_rows(seeded):
 async def test_backfill_makes_a_pre_0070_snippet_findable_by_location(seeded):
     """A snippet stored before migration 0070 has no `data`, so the location query
     can't see it. Unfilled, it reads as "nothing recorded here" — the failure the
-    backfill exists to prevent."""
+    backfill exists to prevent.
+
+    `data` is OMITTED from the constructor on purpose: that leaves a genuine SQL
+    NULL, which is the shape migration 0070 actually left behind. Passing
+    `data=None` would store the JSON encoding of null instead (SQLAlchemy JSON
+    defaults to none_as_null=False) — a different state, covered by the next test.
+    """
     user_id, _ids = seeded
     locations = [_loc("Legacy", "old/path/y.py", "legacy_helper")]
     async with async_session() as s:
@@ -251,7 +257,6 @@ async def test_backfill_makes_a_pre_0070_snippet_findable_by_location(seeded):
             title="legacy — recorded before the data column existed",
             body=compose_body(code="return 2", language="python", locations=locations),
             tags=["python", "snippet"],
-            data=None,
         )
         s.add(old)
         await s.commit()
@@ -274,9 +279,11 @@ async def test_backfill_makes_a_pre_0070_snippet_findable_by_location(seeded):
 
 
 @pytest.mark.asyncio
-async def test_backfill_is_idempotent_and_settles_a_fieldless_snippet(seeded):
-    """Second run fills nothing, and a snippet with no structured fields lands at
-    `{}` rather than staying NULL and being rescanned on every boot."""
+async def test_backfill_fills_a_json_null_and_is_idempotent(seeded):
+    """The OTHER unfilled shape: `data=None` persists JSON `null`, not SQL NULL
+    (SQLAlchemy JSON defaults to none_as_null=False), so an `IS NULL` test alone
+    would walk past this row. Also pins idempotence — a fieldless snippet settles
+    at `{}` instead of staying unfilled and being rescanned on every boot."""
     user_id, _ids = seeded
     async with async_session() as s:
         bare = Note(
@@ -291,7 +298,17 @@ async def test_backfill_is_idempotent_and_settles_a_fieldless_snippet(seeded):
         await s.commit()
         bare_id = bare.id
 
-    await backfill_snippet_data()
+    # Confirm the premise rather than assuming it: this row really is JSON null,
+    # so the assertion below is testing the second arm of the predicate.
+    async with async_session() as s:
+        kind = (
+            await s.execute(
+                select(func.jsonb_typeof(Note.data)).where(Note.id == bare_id)
+            )
+        ).scalar_one()
+        assert kind == "null"
+
+    assert await backfill_snippet_data() >= 1
     assert await backfill_snippet_data() == 0
 
     async with async_session() as s: