Move cover bytes to filesystem storage

backend/internal/store/migrations/0008_filesystem_covers.sql

@@ -0,0 +1,9 @@
+-- Cover bytes move out of Postgres. Existing rows are intentionally dropped:
+-- the old kagane path already refetches missing Covers on demand.
+DROP TABLE covers;
+
+CREATE TABLE covers (
+  address      text PRIMARY KEY,
+  path         text NOT NULL,
+  content_type text NOT NULL
+);