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/tests/test_memory_retrieval.py

@@ -26,7 +26,7 @@ import asyncio
 from datetime import timedelta
 
 import pytest
-from sqlalchemy import event
+from sqlalchemy import event, inspect as sa_inspect
 
 from app import memorybank, models
 from app.database import Base, SessionLocal, engine
@@ -206,13 +206,14 @@ def memory_selects(statements):
     ]
 
 
-def test_the_json_column_is_never_selected(db, adventure, settings, bank, sql_log):
-    """`memories.embedding` is dead weight kept only until a follow-up
-    migration drops it. If anything still reads it, dropping it breaks."""
-    retrieve(adventure, settings, StubEmbedder())
-    offenders = [s for s in memory_selects(sql_log) if "memories.embedding " in s
-                 or s.rstrip().endswith("memories.embedding")]
-    assert offenders == [], f"the JSON column was read:\n{offenders[0][:300]}"
+def test_the_json_column_is_gone(db):
+    """`memories.embedding` held the vectors before migration 38 and nothing
+    read it afterwards; migration 42 dropped it. Bringing it back would restore
+    4 MB of dead weight and a second place vectors can be written from — which
+    is how the model-switch bug happened (test_embedding_model_switch.py)."""
+    columns = {c["name"] for c in sa_inspect(engine).get_columns("memories")}
+    assert "embedding" not in columns
+    assert {"embedding_blob", "embedded"} <= columns
 
 
 def test_the_catalogue_query_carries_no_vectors(db, adventure, settings, bank, sql_log):