Persist kagane covers in Postgres

backend/internal/store/migrations/0007_covers.sql

@@ -0,0 +1,8 @@
+-- Kagane cover bytes belong in their own table so image blobs never enter the
+-- series queries that drive the latest-chapter poller.
+CREATE TABLE covers (
+  image_id    text PRIMARY KEY,
+  body        bytea NOT NULL,
+  content_type text NOT NULL,
+  fetched_at  timestamptz NOT NULL DEFAULT now()
+);