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>

alembic/versions/0109_embeddings_record_their_model.py

@@ -0,0 +1,42 @@
+"""embeddings_record_their_model — a vector says whose space it lives in (#4132)
+
+Revision ID: 0109
+Revises: 0108
+Create Date: 2026-09-23
+
+Every embedding table stamps `chunker_version`, so a change to the document
+shape is caught per row and re-embedded. None stamped the MODEL. The column is
+`vector(384)` — a width, not an identity — so swapping bge-small for any other
+384-dim model would write a second geometry beside the first with no error,
+and search would go on ranking by cosines between the two, which mean nothing.
+
+`embedding_model` is the other half of `calibration_stamp()`, stored per row on
+all four tables. The startup backfill now re-embeds on either half moving.
+
+THE BACKFILL LITERAL IS SAFE BECAUSE NO INSTALL HAS EVER CHANGED MODELS. The
+name is hardcoded in `services/embeddings.py`, and every vector ever written was
+written by it — so stamping every existing row with that name states a fact,
+not a guess. It is frozen here rather than imported: a migration records what
+was true when it ran, and a later model change must not rewrite history.
+"""
+from alembic import op
+
+revision = "0109"
+down_revision = "0108"
+branch_labels = None
+depends_on = None
+
+_TABLES = ("note_embeddings", "rule_embeddings", "milestone_embeddings", "system_embeddings")
+_MODEL = "BAAI/bge-small-en-v1.5"
+
+
+def upgrade() -> None:
+    for table in _TABLES:
+        op.execute(f"ALTER TABLE {table} ADD COLUMN embedding_model text")
+        op.execute(f"UPDATE {table} SET embedding_model = '{_MODEL}'")
+        op.execute(f"ALTER TABLE {table} ALTER COLUMN embedding_model SET NOT NULL")
+
+
+def downgrade() -> None:
+    for table in _TABLES:
+        op.execute(f"ALTER TABLE {table} DROP COLUMN embedding_model")