Drop the JSON vector column, and fix what was hiding behind it

Migration 38 left memories.embedding in place so a rollback could still find
the vectors. Production has since been verified reading from embedding_blob,
so migration 42 drops it: 4 MB of a 99.6 MB database holding nothing anyone
reads.

Removing it surfaced a live bug. Changing your embedding model is supposed to
throw the bank's vectors away and let the post-turn pass rebuild them, because
two models' vectors are not comparable. The settings route did that by nulling
memories.embedding -- correct until 38 moved the vectors, after which it
cleared the dead column and left the blob intact with `embedded` still true.
_embed_pending filters on `embedded IS FALSE`, so it never saw those rows and
the bank went on ranking against the old model's vectors permanently.

Nothing would have reported it. cosine returns 0.0 on a width mismatch, so a
different-width model scores every memory zero and retrieval returns whichever
rows happen to sort first; a same-width model scores plausible garbage.

The bulk clear now sets both columns. It stays a bulk UPDATE rather than going
through set_vector -- loading the rows is the cost that whole path exists to
avoid -- so set_vector's docstring now names it as the one caller that
legitimately writes those columns by hand. No cache invalidation is added:
clearing `embedded` drops the rows out of the catalogue query, and set_vector
evicts each entry as the re-embed puts it back.

test_embedding_blob.py now rebuilds the pre-38 schema by hand where it tests
the backfill, since create_all no longer produces the column it converts from,
and asserts 42 removes it at the end of a full bootstrap -- 38 reads that
column and 42 drops it, so an upgrade that reordered them would arrive with an
empty bank.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017Dvvqn9ZDR4ixeFPHNbww7

backend/app/migrations.py

@@ -144,6 +144,12 @@ MIGRATIONS: list[tuple[int, str | dict[str, str]]] = [
     # so anyone who picked a value keeps it — same rule as migration 29.
     # Adventures already over 80 evict down on their next turn.
     (41, "UPDATE settings SET memory_bank_capacity = 80 WHERE memory_bank_capacity = 200"),
+    # The JSON vectors, gone. Migration 38 left them in place so a rollback
+    # could still find them; production has since been verified reading from
+    # embedding_blob (schema_version 41, 134/134 backfilled), so the column is
+    # now 4 MB of a 99.6 MB database holding nothing anyone reads. DROP COLUMN
+    # is spelled the same on both dialects — SQLite has had it since 3.35.
+    (42, "ALTER TABLE memories DROP COLUMN embedding"),
 ]
 
 LATEST_VERSION = max((v for v, _ in MIGRATIONS), default=1)