feat: add upcoming seasons sync and notification for sequels

migrations/005_add_anime_relations.sql

@@ -0,0 +1,9 @@
+ALTER TABLE anime ADD COLUMN status TEXT DEFAULT '';
+ALTER TABLE anime ADD COLUMN relations_synced_at DATETIME;
+
+CREATE TABLE IF NOT EXISTS anime_relation (
+    anime_id INTEGER NOT NULL REFERENCES anime(id) ON DELETE CASCADE,
+    related_anime_id INTEGER NOT NULL,
+    relation_type TEXT NOT NULL,
+    PRIMARY KEY (anime_id, related_anime_id)
+);