fix: 按完成时刻分批清理并限制读连接池

internal/store/migrations/0004_cleanup_indexes.sql

@@ -0,0 +1,11 @@
+-- C-03：完成时刻列与清理索引（编号避开 U-02 的 0003）
+ALTER TABLE messages ADD COLUMN completed_at INTEGER;
+
+UPDATE messages SET completed_at = COALESCE(
+  (SELECT MAX(d.updated_at) FROM deliveries d WHERE d.seq = messages.seq),
+  send_at
+) WHERE state = 'completed' AND completed_at IS NULL;
+
+CREATE INDEX IF NOT EXISTS idx_messages_completed_at ON messages(completed_at);
+CREATE INDEX IF NOT EXISTS idx_receipts_created ON receipts(created_at);
+CREATE INDEX IF NOT EXISTS idx_send_keys_created ON send_keys(created_at);