feat(snippets): add notes.data JSONB — the indexed mirror of snippet fields

Milestone #232 step 1 (task #2081). Takes the enabler first rather than the
write-path trigger: reverse lookup, drift checks and the duplicate finder all
need to QUERY structured fields, and building them on body-regex first means
writing them twice.

#227 deferred this bag "unless body-convention ergonomics prove insufficient" —
answering "which snippets live in this file?" by scanning every snippet and
regexing its body is that condition being met.

Migration 0070 adds `notes.data` (nullable JSONB) + a GIN index. The body is
UNCHANGED and still what gets embedded and read by humans; `data` mirrors the
same facts in a shape Postgres can index. Code is deliberately not copied into
it — the body holds it, and duplicating a blob into the column we index around
would be waste.

- compose_data() builds the mirror, omitting empties so the column stays sparse
- snippet_fields() prefers `data`, falling back to parsing the body. Rows written
  before 0070 have no `data` and are never backfilled, so a hand-edited body
  stays authoritative for them with no conversion deadline
- create / update / merge all write body and mirror from the same merged field
  set, so the two can't drift; merge in particular has to grow the mirror with
  the survivor's location set or a merged snippet would be unfindable at the very
  call sites the merge just recorded

Named `data`, not `metadata`, because that collides with SQLAlchemy's declarative
Base.metadata — which is why the pre-0069 model had to map an awkward
`entity_metadata` attribute. Not a revival of the column 0069 dropped: different
name, different purpose, nothing reads the old shape.

Two test fakes needed an explicit `data = None`: snippet_fields prefers `data`
when truthy and an auto-MagicMock attribute is truthy, so every parsed field
would have come back a MagicMock. Checked every fake reaching snippet code this
time rather than waiting for CI (note 2109).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01RLwAaV4DQEmVyn496HnEvt

src/scribe/models/note.py

@@ -69,6 +69,13 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
     # notes keep the 'work' default and ignore it. Orthogonal to note_type
     # (which is the note/entity axis).
     task_kind: Mapped[str] = mapped_column(Text, default="work", server_default="work")
+    # Queryable structured fields for typed records — currently snippets, whose
+    # name/language/signature/locations live here so they can be INDEXED. The
+    # body keeps the same facts in readable markdown and remains what gets
+    # embedded; this is a mirror for querying, not the source of truth for
+    # display. NULL on every row written before migration 0070, so readers fall
+    # back to parsing the body (see services/snippets.snippet_fields).
+    data: Mapped[dict | None] = mapped_column(JSONB, nullable=True)
 
     __table_args__ = (
         Index("ix_notes_tags", "tags", postgresql_using="gin"),
@@ -79,6 +86,9 @@ class Note(Base, TimestampMixin, SoftDeleteMixin):
         Index("ix_notes_milestone_id", "milestone_id"),
         Index("ix_notes_note_type", "note_type"),
         Index("ix_notes_arose_from_id", "arose_from_id"),
+        # Containment queries into `data` — e.g. which snippets name a given
+        # repo/path in their locations. See migration 0070.
+        Index("ix_notes_data_gin", "data", postgresql_using="gin"),
     )
 
     @property