Fix backend to return empty slices instead of null for JSON encoding

- Initialize slices as empty in database query functions to prevent json.Encode(nil) returning 'null'
- This fixes Flutter Dio parsing 'null' as null, causing cast to List to fail

go_cloud/internal/database/db.go

@@ -349,10 +349,10 @@ func (db *DB) GetOrgMembersWithUsers(ctx context.Context, orgID uuid.UUID) ([]st
 	}
 	defer rows.Close()
 
-	var members []struct {
+	members := make([]struct {
 		Membership
 		User
-	}
+	}, 0)
 	for rows.Next() {
 		var m struct {
 			Membership
@@ -406,7 +406,7 @@ func (db *DB) SearchUsersByUsername(ctx context.Context, query string, limit int
 	}
 	defer rows.Close()
 
-	var users []User
+	users := make([]User, 0)
 	for rows.Next() {
 		var u User
 		err := rows.Scan(&u.ID, &u.Email, &u.Username, &u.DisplayName, &u.CreatedAt, &u.LastLoginAt)
@@ -448,7 +448,7 @@ func (db *DB) GetOrgInvitations(ctx context.Context, orgID uuid.UUID) ([]Invitat
 	}
 	defer rows.Close()
 
-	var invitations []Invitation
+	invitations := make([]Invitation, 0)
 	for rows.Next() {
 		var inv Invitation
 		err := rows.Scan(
@@ -510,10 +510,10 @@ func (db *DB) GetOrgJoinRequests(ctx context.Context, orgID uuid.UUID) ([]struct
 	}
 	defer rows.Close()
 
-	var requests []struct {
+	requests := make([]struct {
 		JoinRequest
 		User
-	}
+	}, 0)
 	for rows.Next() {
 		var r struct {
 			JoinRequest