feat: add affiliate invite rebate flow and admin rebate-rate setting

backend/migrations/130_add_user_affiliates.sql

@@ -0,0 +1,20 @@
+CREATE TABLE IF NOT EXISTS user_affiliates (
+    user_id BIGINT PRIMARY KEY REFERENCES users(id) ON DELETE CASCADE,
+    aff_code VARCHAR(32) NOT NULL UNIQUE,
+    inviter_id BIGINT NULL REFERENCES users(id) ON DELETE SET NULL,
+    aff_count INTEGER NOT NULL DEFAULT 0,
+    aff_quota DECIMAL(20,8) NOT NULL DEFAULT 0,
+    aff_history_quota DECIMAL(20,8) NOT NULL DEFAULT 0,
+    created_at TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at TIMESTAMPTZ NOT NULL DEFAULT NOW()
+);
+
+CREATE INDEX IF NOT EXISTS idx_user_affiliates_inviter_id ON user_affiliates(inviter_id);
+CREATE INDEX IF NOT EXISTS idx_user_affiliates_aff_quota ON user_affiliates(aff_quota);
+
+COMMENT ON TABLE user_affiliates IS '用户邀请返利信息';
+COMMENT ON COLUMN user_affiliates.aff_code IS '用户邀请代码';
+COMMENT ON COLUMN user_affiliates.inviter_id IS '邀请人用户ID';
+COMMENT ON COLUMN user_affiliates.aff_count IS '累计邀请人数';
+COMMENT ON COLUMN user_affiliates.aff_quota IS '当前可提取返利金额';
+COMMENT ON COLUMN user_affiliates.aff_history_quota IS '累计返利历史金额';