feat(search): milestones are searchable by meaning — "is there already a plan for this?" (#4078)

`search` covered notes, tasks and rules, and a milestone — the record a plan
lives in — could not be found. A project whose roadmap was written as
milestones had every later plan opened beside the one that already described
it, because nothing could have told the session it existed.

- milestone_embeddings (migration 0102): the third sibling of note_ and
  rule_embeddings, for note 3163's reason — the search is milestone-specific.
  The document is title — description, then description and the plan body,
  so a roadmap milestone with no description is still found by its design.
- Written on create, on a title/description/body update, and for a plan made
  through start_planning / create_records, fire-and-forget with the parent-row
  claim (#3262); a startup backfill covers every existing milestone. Derived,
  so it joins _NOT_INCLUDED beside the other embeddings.
- semantic_search_milestones: a project's milestones when the caller can read
  it (access.can_read_project), otherwise the caller's own; optional status.
- search(content_type="milestone"): id, title, description, status, project
  and progress. Its own shape, and not part of "all", whose results are
  note-shaped. The docstring says what it is for: ask before start_planning.
- Integration test on real Postgres: found in its project and not another,
  status narrows, an unreadable project returns nothing.

Milestone 415 step 3.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01821k5B3Ysecp9fNYs92Kuy

src/scribe/services/milestones.py

@@ -11,6 +11,30 @@ from scribe.models.note import Note
 logger = logging.getLogger(__name__)
 
 
+def embed_milestone(milestone: Milestone) -> None:
+    """Refresh a milestone's vectors, fire-and-forget (milestone 415).
+
+    At the service, so every path that writes a milestone gets it — the lesson
+    embed_note records (#2056): a record written through a door that forgot the
+    call stays out of search until a restart. Exceptions are swallowed because
+    a milestone that saved must not fail on its index refresh; no running loop
+    (a script, a unit test) is ordinary. A delete racing the refresh wins: the
+    upsert claims the milestone's row first (#3262).
+    """
+    try:
+        import asyncio
+
+        from scribe.services.embeddings import upsert_milestone_embedding
+
+        asyncio.create_task(upsert_milestone_embedding(
+            milestone.id, milestone.title, milestone.description, milestone.body,
+        ))
+    except RuntimeError:
+        pass
+    except Exception:  # noqa: BLE001 - never let indexing break a write
+        logger.exception("embedding refresh failed for milestone %s", milestone.id)
+
+
 async def create_milestone(
     user_id: int,
     project_id: int,
@@ -33,6 +57,7 @@ async def create_milestone(
         session.add(milestone)
         await session.commit()
         await session.refresh(milestone)
+        embed_milestone(milestone)
         return milestone
 
 
@@ -125,6 +150,8 @@ async def update_milestone(user_id: int, milestone_id: int, **fields: object) ->
         milestone.updated_at = datetime.now(timezone.utc)
         await session.commit()
         await session.refresh(milestone)
+        if {"title", "description", "body"} & set(fields):
+            embed_milestone(milestone)
         return milestone