feat: 支持用户专属分组倍率配置

backend/migrations/047_add_user_group_rate_multipliers.sql

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+-- 用户专属分组倍率表
+-- 允许管理员为特定用户设置分组的专属计费倍率，覆盖分组默认倍率
+CREATE TABLE IF NOT EXISTS user_group_rate_multipliers (
+    user_id         BIGINT NOT NULL REFERENCES users(id) ON DELETE CASCADE,
+    group_id        BIGINT NOT NULL REFERENCES groups(id) ON DELETE CASCADE,
+    rate_multiplier DECIMAL(10,4) NOT NULL,
+    created_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    updated_at      TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+    PRIMARY KEY (user_id, group_id)
+);
+
+-- 按 group_id 查询索引（删除分组时清理关联记录）
+CREATE INDEX IF NOT EXISTS idx_user_group_rate_multipliers_group_id
+    ON user_group_rate_multipliers(group_id);
+
+COMMENT ON TABLE user_group_rate_multipliers IS '用户专属分组倍率配置';
+COMMENT ON COLUMN user_group_rate_multipliers.user_id IS '用户ID';
+COMMENT ON COLUMN user_group_rate_multipliers.group_id IS '分组ID';
+COMMENT ON COLUMN user_group_rate_multipliers.rate_multiplier IS '专属计费倍率（覆盖分组默认倍率）';