番組登録を追加（登録時の過去回の選択、一時停止・再開・解除、既知の回の記録）

internal/store/migrations/002_known_episodes.sql

@@ -0,0 +1,8 @@
+CREATE TABLE known_episodes (
+    series_id TEXT NOT NULL REFERENCES subscriptions(series_id) ON DELETE CASCADE,
+    episode_id TEXT NOT NULL,
+    first_seen_at TIMESTAMPTZ NOT NULL DEFAULT now(),
+    PRIMARY KEY (series_id, episode_id)
+);
+
+CREATE INDEX download_jobs_series_idx ON download_jobs (series_id);