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/alembic/versions/0100_drop_plex_item_markers.py

@@ -0,0 +1,22 @@
+"""plex_items: drop `markers`, a column nothing writes or reads
+
+Intro/credits markers are read live from Plex for the item the player opens (with Plex's `final` flag
+on the block that ends it); this cached copy was never filled — every row is NULL. The downgrade
+restores the empty column.
+"""
+
+import sqlalchemy as sa
+from alembic import op
+
+revision: str = "0100_drop_plex_item_markers"
+down_revision: str | None = "0099_library_states"
+branch_labels: str | None = None
+depends_on: str | None = None
+
+
+def upgrade() -> None:
+    op.drop_column("plex_items", "markers")
+
+
+def downgrade() -> None:
+    op.add_column("plex_items", sa.Column("markers", sa.JSON(), nullable=True))