feat(embeddings): per-chunk rows — schema, write path, version-aware backfill (#280 steps 2+3)

note_embeddings becomes one row per chunk: PK (note_id, chunk_index), plus
chunk_text (what this vector actually encodes) and chunker_version. Migration
0077 clears the table — embeddings are derived (0067 precedent) and the old
whole-document rows are indistinguishable from single-chunk notes, so the
startup backfill regenerates the corpus at the new shape. The backfill is now
version-aware: a future shape change is a CHUNKER_VERSION bump that re-embeds
exactly the stale notes, not another wipe.

upsert_note_embedding takes (title, body) and chunks internally — one path
for the write path, the recurrence spawn and the backfill. The recurrence
spawn's own embed call is deleted outright: create_note already embeds via
embed_note (#2056), so the spawn was a second copy of the rule. An emptied
record now CLEARS its stale vectors instead of leaving them findable.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01UaYUaouG9jjhATyuxCKrQs

tests/test_services_notes.py

@@ -29,18 +29,25 @@ def test_embed_note_uses_the_OWNER_not_the_caller():
     upsert.assert_called_once()
     assert upsert.call_args.args[0] == 5
     assert upsert.call_args.args[1] == 42          # owner, never the caller
-    assert upsert.call_args.args[2] == "T\nB"
+    # Title and body travel separately since #280 — chunking happens inside
+    # upsert_note_embedding, the one path every writer shares.
+    assert upsert.call_args.args[2] == "T"
+    assert upsert.call_args.args[3] == "B"
 
 
-def test_embed_note_skips_a_record_with_no_text():
-    """An empty embedding is worse than none — it is a row that matches nothing
-    and hides the fact that the record was never indexed."""
+def test_embed_note_hands_even_an_empty_record_to_the_one_path():
+    """The empty-record decision moved INTO upsert_note_embedding (#280): an
+    emptied record must have its stale vectors CLEARED, not merely skipped —
+    so embed_note schedules the call unconditionally rather than deciding
+    here. The clearing behaviour itself is pinned in test_chunking.py."""
     from unittest.mock import MagicMock, patch
 
     note = MagicMock(id=5, user_id=42, title="", body="")
-    with patch("asyncio.create_task") as create_task:
+    with patch("scribe.services.embeddings.upsert_note_embedding") as upsert, \
+         patch("asyncio.create_task") as create_task:
         notes_svc.embed_note(note)
-    assert not create_task.called
+    assert create_task.called
+    upsert.assert_called_once()
 
 
 def test_embed_note_without_a_running_loop_is_not_an_error():