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_integration_pgvector_search.py

@@ -31,6 +31,20 @@ def _vec(*nonzero_first):
     return v[:EMBEDDING_DIM]
 
 
+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
+
+    return NoteEmbedding(
+        note_id=note_id,
+        chunk_index=chunk_index,
+        user_id=user_id,
+        embedding=vec,
+        chunk_text=f"chunk {chunk_index} of note {note_id}",
+        chunker_version=CHUNKER_VERSION,
+    )
+
+
 @pytest_asyncio.fixture(autouse=True)
 async def _dispose_engine():
     # Per-loop pool: dispose after each test (see test_integration_db_maintenance).
@@ -54,8 +68,8 @@ async def seeded():
         await s.flush()
         # query vector will be [1,0,0,...]; near ~ identical (sim≈1.0),
         # far is orthogonal (sim≈0.0 -> filtered by the default threshold).
-        s.add(NoteEmbedding(note_id=near.id, user_id=user.id, embedding=_vec(1.0)))
-        s.add(NoteEmbedding(note_id=far.id, user_id=user.id, embedding=_vec(0.0, 1.0)))
+        s.add(_emb(near.id, user.id, 0, _vec(1.0)))
+        s.add(_emb(far.id, user.id, 0, _vec(0.0, 1.0)))
         await s.commit()
         ids = (user.id, near.id, far.id)
     yield ids