feat(embeddings): every vector records the model whose space it lives in (#4132)

The four embedding tables stamped chunker_version but not the model, and
vector(384) is a width, not an identity: a same-width model swap would
write a second geometry beside the first with no error.

- embedding_model on note/rule/milestone/system embeddings (0109; existing
  rows stamped with the only model any install has ever run).
- Every write stamps EMBEDDING_MODEL; every backfill's "current" test is
  is_current_stamp(), both halves of calibration_stamp().
- migrate_floor refuses while any row its surface searches is off the live
  model, before sampling: re-embed, then migrate.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_integration_pgvector_search.py

@@ -33,7 +33,7 @@ def _vec(*nonzero_first):
 
 def _emb(note_id, user_id, chunk_index, vec):
     """A chunk row at the current chunker version (#280, migration 0077)."""
-    from scribe.services.embeddings import CHUNKER_VERSION
+    from scribe.services.embeddings import CHUNKER_VERSION, EMBEDDING_MODEL
 
     return NoteEmbedding(
         note_id=note_id,
@@ -42,6 +42,7 @@ def _emb(note_id, user_id, chunk_index, vec):
         embedding=vec,
         chunk_text=f"chunk {chunk_index} of note {note_id}",
         chunker_version=CHUNKER_VERSION,
+        embedding_model=EMBEDDING_MODEL,
     )