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>

alembic/versions/0108_trigger_leaves_the_stored_title.py

@@ -0,0 +1,64 @@
+"""trigger_leaves_the_stored_title — a snippet's and lesson's title is its name (milestone 427)
+
+Revision ID: 0108
+Revises: 0107
+Create Date: 2026-09-23
+
+Snippets and lessons stored their title as `subject — trigger`, because the
+join is what makes these kinds rank on the situation they apply to (#2485) and
+the stored title WAS the embedded one. Every surface that shows a title then
+showed the trigger too: menu lines, lists and search rows ran to 1,500–3,000
+characters, and an injected repeat spent all of it again.
+
+The trigger's home is `data` (decision #4157), and since this milestone the
+join happens at embed time (`embeddings.document_title`). This revision
+rewrites the rows already stored.
+
+EXACT-SUFFIX, READ FROM THE ROW'S OWN MIRROR. A title is only rewritten when
+it ends with `' — ' || <that row's trigger>`, so a subject that legitimately
+contains an em dash is never cut, and a row whose title and mirror disagree —
+hand-edited through the generic note door — is left alone. Such a row still
+embeds correctly (`document_title` is idempotent) and merely shows its old
+title; that is the right way round for a data migration to fail.
+
+NO RE-EMBED, AND `updated_at` IS NOT TOUCHED. The embedded text is identical
+before and after, so the vectors are already right. The startup backfill
+re-embeds any row whose `updated_at` is newer than its vectors, and a raw
+UPDATE that leaves `updated_at` alone is what keeps this from queueing the
+whole snippet corpus for work that would produce the same numbers. There is
+no database trigger on `updated_at`; it is set by the ORM only.
+"""
+from alembic import op
+
+revision = "0108"
+down_revision = "0107"
+branch_labels = None
+depends_on = None
+
+# kind → the `data` key its trigger is mirrored under (embeddings._TRIGGER_DATA_KEYS).
+_KINDS = (("snippet", "when_to_use"), ("lesson", "when_to_apply"))
+_SEP = " — "
+
+
+def upgrade() -> None:
+    for note_type, key in _KINDS:
+        op.execute(f"""
+            UPDATE notes
+               SET title = btrim(left(title, length(title) - length('{_SEP}' || (data->>'{key}'))))
+             WHERE note_type = '{note_type}'
+               AND coalesce(btrim(data->>'{key}'), '') <> ''
+               AND right(title, length('{_SEP}' || (data->>'{key}'))) = '{_SEP}' || (data->>'{key}')
+        """)
+
+
+def downgrade() -> None:
+    # Recompose, on the same condition inverted: only where the trigger is not
+    # already on the end, so a downgrade run twice cannot double it.
+    for note_type, key in _KINDS:
+        op.execute(f"""
+            UPDATE notes
+               SET title = title || '{_SEP}' || (data->>'{key}')
+             WHERE note_type = '{note_type}'
+               AND coalesce(btrim(data->>'{key}'), '') <> ''
+               AND right(title, length('{_SEP}' || (data->>'{key}'))) <> '{_SEP}' || (data->>'{key}')
+        """)