feat(server): 迁移 000021 pay_purchases 台账 + subscriptions.source 扩 pay

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

server/migrations/sqlite/000021_pay_purchases.down.sql

@@ -0,0 +1,16 @@
+DROP TABLE IF EXISTS pay_purchases;
+CREATE TABLE subscriptions_old (
+  id          INTEGER PRIMARY KEY AUTOINCREMENT,
+  user_id     INTEGER NOT NULL,
+  plan_id     INTEGER NOT NULL,
+  expires_at  DATETIME NOT NULL,
+  source      TEXT NOT NULL CHECK (source IN ('trial','code')),
+  created_at  DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+  FOREIGN KEY (user_id) REFERENCES users(id),
+  FOREIGN KEY (plan_id) REFERENCES plans(id)
+);
+INSERT INTO subscriptions_old (id, user_id, plan_id, expires_at, source, created_at)
+  SELECT id, user_id, plan_id, expires_at, source, created_at FROM subscriptions;
+DROP TABLE subscriptions;
+ALTER TABLE subscriptions_old RENAME TO subscriptions;
+CREATE INDEX idx_subs_user_exp ON subscriptions (user_id, expires_at);