feat(server/auth): switch user identity to phone with email optional

Phone becomes the required, unique login identifier; email and name turn
optional. Login now accepts either via a single identifier field, refresh
re-checks tenant membership, and the dev seed provides phone numbers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

server/internal/tenant/domain/user.go

@@ -4,10 +4,10 @@ import "time"
 
 type User struct {
 	ID           int64
-	Email        string
-	Phone        *string
+	Email        *string
+	Phone        string
 	PasswordHash string
-	Name         string
+	Name         *string
 	AvatarURL    *string
 	Status       string
 	CreatedAt    time.Time