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>

src/scribe/services/notes.py

@@ -115,11 +115,17 @@ def embed_note(note) -> None:
     try:
         import asyncio
 
-        from scribe.services.embeddings import upsert_note_embedding
+        from scribe.services.embeddings import document_title, upsert_note_embedding
         # Chunking and the empty-record gate live inside upsert_note_embedding —
-        # one path for every writer (#280).
+        # one path for every writer (#280). The title is the EMBEDDED one: a
+        # snippet's or lesson's trigger joins its name here, not in the stored
+        # title (milestone 427).
         asyncio.create_task(
-            upsert_note_embedding(note.id, note.user_id, note.title, note.body)
+            upsert_note_embedding(
+                note.id, note.user_id,
+                document_title(note.title, note.note_type, note.data, note.body),
+                note.body,
+            )
         )
     except RuntimeError:
         pass          # no running loop — a sync caller, not a failure