feat(payment): Stripe Checkout payment module

- payment_orders 建表迁移，支持多 provider（stripe）
- Stripe Checkout Session 创建、Webhook 验签、到账
- 支付成功自动为 API Key 增加额度
- payment.enabled 开关，关闭时不影响旧 Comfy Checkout

migrations/20260610000001_payment_orders.sql

@@ -0,0 +1,39 @@
+CREATE TABLE IF NOT EXISTS payment_orders (
+    id                  INTEGER PRIMARY KEY AUTOINCREMENT NOT NULL,
+    public_id           TEXT NOT NULL,
+    user_id             INTEGER NOT NULL,
+    recharge_record_id  INTEGER,
+    provider            TEXT NOT NULL,
+    payment_method      TEXT NOT NULL DEFAULT 'stripe',
+    provider_session_id TEXT,
+    provider_payment_id TEXT,
+    amount_usd          REAL NOT NULL,
+    amount_minor        INTEGER NOT NULL,
+    currency            TEXT NOT NULL DEFAULT 'usd',
+    status              TEXT NOT NULL DEFAULT 'pending',
+    checkout_url        TEXT,
+    success_url         TEXT,
+    cancel_url          TEXT,
+    note                TEXT,
+    metadata            TEXT,
+    created_at          TEXT DEFAULT (datetime('now')),
+    updated_at          TEXT DEFAULT (datetime('now')),
+    paid_at             TEXT,
+    completed_at        TEXT,
+    expires_at          TEXT,
+    FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE,
+    FOREIGN KEY (recharge_record_id) REFERENCES user_recharge_records(id) ON DELETE SET NULL
+);
+
+CREATE UNIQUE INDEX IF NOT EXISTS payment_orders_public_id_unique
+    ON payment_orders (public_id);
+
+CREATE UNIQUE INDEX IF NOT EXISTS payment_orders_provider_session_unique
+    ON payment_orders (provider, provider_session_id)
+    WHERE provider_session_id IS NOT NULL;
+
+CREATE INDEX IF NOT EXISTS payment_orders_user_created_idx
+    ON payment_orders (user_id, created_at DESC);
+
+CREATE INDEX IF NOT EXISTS payment_orders_status_idx
+    ON payment_orders (status);