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/models/__init__.py

@@ -25,7 +25,7 @@ from scribe.models.user import User  # noqa: E402, F401
 from scribe.models.app_log import AppLog  # noqa: E402, F401
 from scribe.models.password_reset import PasswordResetToken  # noqa: E402, F401
 from scribe.models.invitation import InvitationToken  # noqa: E402, F401
-from scribe.models.embedding import NoteEmbedding, RuleEmbedding  # noqa: E402, F401
+from scribe.models.embedding import MilestoneEmbedding, NoteEmbedding, RuleEmbedding  # noqa: E402, F401
 from scribe.models.retrieval_log import RetrievalLog  # noqa: E402, F401
 from scribe.models.note_usage import NoteUsageEvent  # noqa: E402, F401
 from scribe.models.rule_usage import RuleUsageEvent  # noqa: E402, F401

src/scribe/models/embedding.py

@@ -95,3 +95,36 @@ class RuleEmbedding(Base):
         DateTime(timezone=True),
         default=lambda: datetime.now(timezone.utc),
     )
+
+
+class MilestoneEmbedding(Base):
+    """One embedding vector per CHUNK of a milestone (milestone 415).
+
+    The third sibling, for note 3163's reason: the row could be shared, the
+    search cannot. A milestone is scoped by its project, has no share of its
+    own, and is searched to answer one question — "is there already a plan for
+    this?" — which no note or rule search can answer, because a plan is not a
+    note. Before this, a roadmap written as milestones was invisible to recall,
+    and every later plan was opened as a new milestone beside the one that
+    already described it.
+
+    The document is the title, the one-line description and the plan body, the
+    parts a reader uses to recognise a plan. Derived data: the startup backfill
+    regenerates it, which is also how a chunker-version bump is handled.
+    """
+
+    __tablename__ = "milestone_embeddings"
+
+    milestone_id: Mapped[int] = mapped_column(
+        Integer,
+        ForeignKey("milestones.id", ondelete="CASCADE"),
+        primary_key=True,
+    )
+    chunk_index: Mapped[int] = mapped_column(Integer, primary_key=True)
+    embedding: Mapped[list] = mapped_column(Vector(EMBEDDING_DIM), nullable=False)
+    chunk_text: Mapped[str] = mapped_column(Text, nullable=False)
+    chunker_version: Mapped[int] = mapped_column(Integer, nullable=False)
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        default=lambda: datetime.now(timezone.utc),
+    )