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)
                     )
                 )