Support both email and phone login

Auto-detect whether the user entered an email or phone number.
Email sends via Resend, phone sends via Twilio SMS. Users table
has nullable phone and email columns; verification_codes uses a
generic identifier field.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

db/models.go

@@ -55,15 +55,16 @@ type Slot struct {
 
 type User struct {
 	ID        int64
-	Phone     string
+	Phone     sql.NullString
+	Email     sql.NullString
 	Name      string
 	CreatedAt time.Time
 }
 
 type VerificationCode struct {
-	ID        int64
-	Phone     string
-	Code      string
-	ExpiresAt time.Time
-	Used      int64
+	ID         int64
+	Identifier string
+	Code       string
+	ExpiresAt  time.Time
+	Used       int64
 }