chore(db): drop plex_items.markers, a column nothing writes or reads

Markers are read live from Plex for the item the player opens; the cached copy was never filled.

backend/app/models.py

@@ -1274,9 +1274,6 @@ class PlexItem(Base, TimestampMixin, UpdatedAtMixin):
     # bit-depth/HDR. NULL = never (the backfill's work-list). Only HW-decodable transcode items are
     # probed (the rest are SW-only by codec and need no bit-depth); see `_probe_media_batch`.
     media_probed_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
-    # Intro/credit markers: [{"type": "intro"|"credits", "start_s": int, "end_s": int}, ...] — unused:
-    # the player reads them live from Plex (`routes/plex/detail.item_detail`, with Plex's `final`).
-    markers: Mapped[list | None] = mapped_column(JSON)
     added_at: Mapped[datetime | None] = mapped_column(DateTime(timezone=True), index=True)
     # Filterable / orderable metadata mirrored from the Plex section listing (no extra API calls;
     # `rating` = Plex audienceRating, which is ~the IMDb score). The arrays back genre/person/studio