feat: implement AI gateway phase one runtime

apps/api/internal/store/user_group_policy.go

@@ -0,0 +1,41 @@
+package store
+
+import (
+	"context"
+
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/auth"
+	"github.com/jackc/pgx/v5"
+)
+
+type UserGroupPolicy struct {
+	ID                    string
+	GroupKey              string
+	RateLimitPolicy       map[string]any
+	BillingDiscountPolicy map[string]any
+}
+
+func (s *Store) ResolveUserGroupPolicy(ctx context.Context, user *auth.User) (UserGroupPolicy, error) {
+	userGroupID := ""
+	if user != nil {
+		userGroupID = user.UserGroupID
+	}
+	var item UserGroupPolicy
+	var rateLimit []byte
+	var billing []byte
+	err := s.pool.QueryRow(ctx, `
+SELECT id::text, group_key, rate_limit_policy, billing_discount_policy
+FROM gateway_user_groups
+WHERE status = 'active'
+  AND (($1 <> '' AND id = NULLIF($1, '')::uuid) OR ($1 = '' AND group_key = 'default'))
+ORDER BY CASE WHEN id::text = $1 THEN 0 ELSE 1 END, priority ASC
+LIMIT 1`, userGroupID).Scan(&item.ID, &item.GroupKey, &rateLimit, &billing)
+	if err != nil {
+		if err == pgx.ErrNoRows {
+			return UserGroupPolicy{}, nil
+		}
+		return UserGroupPolicy{}, err
+	}
+	item.RateLimitPolicy = decodeObject(rateLimit)
+	item.BillingDiscountPolicy = decodeObject(billing)
+	return item, nil
+}