feat: warn when an index is queried by a different embedding model

Changing EMBEDDINGS_NAME on a populated index is the one failure the width
check cannot catch. Two models of the same width -- mpnet and granite are both
768 -- swap without raising anything, and every query is then embedded by a
different model than the stored vectors were. Nothing fails; answers just get
worse.

Boot now compares what each source was built with against the active model and
names the mismatched sources and the command that fixes them. The comparison
goes through the registry rather than string equality, so a stored alias is not
read as a different model. A source with no recorded model pre-dates the column
and is therefore the legacy model, not unknown.

That check is only as good as sources.model, which reembed was not maintaining:
it rewrote the vectors and left the column naming the old model, so a source
would be reported stale immediately after being migrated. It is now stamped
after each source succeeds, from its own session -- sources lives in the
user-data database while the vectors may not.

tests/scripts/test_reembed.py

@@ -390,3 +390,41 @@ class TestEmbedsInProcess:
         ) or 0):
             assert reembed.main(["--dry-run"]) == 0
         assert seen["delegating"] is False
+
+
+class TestRecordsTheModel:
+    """``sources.model`` is what the boot mismatch check reads."""
+
+    def test_a_re_embedded_source_is_stamped(self, monkeypatch):
+        from application.core.settings import settings
+
+        monkeypatch.setattr(settings, "EMBEDDINGS_NAME", "new/model", raising=False)
+        conn = MagicMock()
+        session = MagicMock()
+        session.__enter__ = MagicMock(return_value=conn)
+        session.__exit__ = MagicMock(return_value=False)
+        with patch("application.storage.db.session.db_session", return_value=session):
+            reembed.record_source_model("src-1")
+        params = conn.execute.call_args.args[1]
+        assert params == {"model": "new/model", "id": "src-1"}
+
+    def test_a_dry_run_stamps_nothing(self, monkeypatch):
+        with patch.object(reembed, "record_source_model") as record, patch.object(
+            reembed, "list_source_ids", return_value=["a"]
+        ), patch.object(reembed, "reembed_pgvector", return_value=(3, 0)):
+            reembed.run("pgvector", None, 64, True)
+        record.assert_not_called()
+
+    def test_a_real_run_stamps_each_source(self, monkeypatch):
+        with patch.object(reembed, "record_source_model") as record, patch.object(
+            reembed, "list_source_ids", return_value=["a", "b"]
+        ), patch.object(reembed, "reembed_pgvector", return_value=(3, 3)):
+            reembed.run("pgvector", None, 64, False)
+        assert [c.args[0] for c in record.call_args_list] == ["a", "b"]
+
+    def test_a_failed_source_is_not_stamped(self):
+        with patch.object(reembed, "record_source_model") as record, patch.object(
+            reembed, "list_source_ids", return_value=["a"]
+        ), patch.object(reembed, "reembed_pgvector", side_effect=RuntimeError("boom")):
+            assert reembed.run("pgvector", None, 64, False) == 1
+        record.assert_not_called()