feat(infra): add channel_config_snapshots migration

k-tv-backend/migrations_sqlite/20260319000000_add_config_snapshots.sql

@@ -0,0 +1,12 @@
+CREATE TABLE channel_config_snapshots (
+    id          TEXT    PRIMARY KEY NOT NULL,
+    channel_id  TEXT    NOT NULL REFERENCES channels(id) ON DELETE CASCADE,
+    config_json TEXT    NOT NULL,
+    version_num INTEGER NOT NULL,
+    label       TEXT,
+    created_at  TEXT    NOT NULL,
+    UNIQUE (channel_id, version_num)
+);
+
+CREATE INDEX idx_config_snapshots_channel
+    ON channel_config_snapshots(channel_id, version_num DESC);