refactor(notes): a snippet's and lesson's stored title is its name; the trigger joins it only in the embedded document (milestone 427)

The title was `subject — trigger` because the stored title WAS the
embedded one, and the join is what makes these kinds rank on the
situation they apply to (#2485). Every surface that shows a title then
showed the trigger too -- menus, lists and search rows ran to kilobytes.

- embeddings.document_title(title, note_type, data, body) joins the
  trigger from `data` (body fallback) at embed time. Idempotent: an
  un-migrated composed title comes out the same, never doubled. The
  embed path, the startup backfill and the dedup gate's semantic signal
  all use it, so the embedded text -- and every vector -- is unchanged.
- Writers store the subject: snippet create/update (service, REST, MCP)
  and lesson_document. Both compose_title helpers are removed.
- Readers: dedup takes `data`; the menus strip the embedded title from a
  passage; list rows project `when_to_use`, which SnippetListView reads.
- 0108 rewrites existing rows on an exact `' — ' || <own trigger>`
  suffix with raw SQL, leaving updated_at alone so the backfill does not
  re-embed the corpus for identical vectors. Downgrade recomposes.

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

tests/test_backfill_reads_current_text.py

@@ -56,7 +56,7 @@ async def test_the_backfill_embeds_the_text_as_it_is_now_not_as_it_was_scanned()
     with (
         patch.object(emb, "async_session", return_value=_ctx(scan)),
         patch.object(emb, "_current_row",
-                     AsyncMock(return_value=(42, "T", *edited))),
+                     AsyncMock(return_value=(42, "T", *edited, "note", None))),
         patch.object(emb, "upsert_note_embedding", AsyncMock()) as upsert,
         patch.object(emb.asyncio, "sleep", AsyncMock()),
     ):
@@ -75,7 +75,7 @@ async def test_a_record_deleted_between_the_scan_and_the_loop_is_skipped():
     with (
         patch.object(emb, "async_session", return_value=_ctx(scan)),
         patch.object(emb, "_current_row",
-                     AsyncMock(side_effect=[None, (42, "T", "body")])),
+                     AsyncMock(side_effect=[None, (42, "T", "body", "note", None)])),
         patch.object(emb, "upsert_note_embedding", AsyncMock()) as upsert,
         patch.object(emb.asyncio, "sleep", AsyncMock()),
     ):
@@ -99,7 +99,7 @@ async def test_a_record_whose_text_outran_its_vectors_is_re_embedded():
 
     with (
         patch.object(emb, "async_session", return_value=_ctx(scan)),
-        patch.object(emb, "_current_row", AsyncMock(return_value=(42, "T", "b"))),
+        patch.object(emb, "_current_row", AsyncMock(return_value=(42, "T", "b", "note", None))),
         patch.object(emb, "upsert_note_embedding", AsyncMock()) as upsert,
         patch.object(emb.asyncio, "sleep", AsyncMock()),
     ):
@@ -119,7 +119,7 @@ async def test_a_task_logged_since_its_vectors_is_re_embedded():
 
     with (
         patch.object(emb, "async_session", return_value=_ctx(scan)),
-        patch.object(emb, "_current_row", AsyncMock(return_value=(42, "T", "b"))),
+        patch.object(emb, "_current_row", AsyncMock(return_value=(42, "T", "b", "note", None))),
         patch.object(emb, "upsert_note_embedding", AsyncMock()) as upsert,
         patch.object(emb.asyncio, "sleep", AsyncMock()),
     ):