feat(schema): add note_version.pin_kind and pin_label

Spec: docs/superpowers/specs/2026-05-13-note-version-pinning-design.md

- pin_kind: NULL=rolling, 'auto'=stability-scan, 'manual'=user-declared.
- pin_label: NULL for rolling; auto-generated for 'auto'; user-supplied
  string for 'manual' (may be NULL).

No backfill — every existing row stays rolling. The daily auto-pin scan
will catch up on the first run after deploy.

src/fabledassistant/models/note_version.py

@@ -14,6 +14,8 @@ class NoteVersion(Base, CreatedAtMixin):
     body: Mapped[str] = mapped_column(Text)
     title: Mapped[str] = mapped_column(Text, default="")
     tags: Mapped[list[str]] = mapped_column(ARRAY(Text), default=list)
+    pin_kind: Mapped[str | None] = mapped_column(Text, nullable=True)
+    pin_label: Mapped[str | None] = mapped_column(Text, nullable=True)
 
     def to_dict(self, include_body: bool = True) -> dict:
         d: dict = {
@@ -22,6 +24,8 @@ class NoteVersion(Base, CreatedAtMixin):
             "user_id": self.user_id,
             "title": self.title,
             "tags": self.tags or [],
+            "pin_kind": self.pin_kind,
+            "pin_label": self.pin_label,
             "created_at": self.created_at.isoformat(),
         }
         if include_body: