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>

schema.sql

@@ -38,7 +38,8 @@ CREATE TABLE IF NOT EXISTS rsvps (
 
 CREATE TABLE IF NOT EXISTS users (
     id         INTEGER PRIMARY KEY AUTOINCREMENT,
-    phone      TEXT    NOT NULL UNIQUE,
+    phone      TEXT    UNIQUE,
+    email      TEXT    UNIQUE,
     name       TEXT    NOT NULL DEFAULT '',
     created_at DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP
 );
@@ -51,7 +52,7 @@ CREATE TABLE IF NOT EXISTS sessions (
 
 CREATE TABLE IF NOT EXISTS verification_codes (
     id         INTEGER PRIMARY KEY AUTOINCREMENT,
-    phone      TEXT    NOT NULL,
+    identifier TEXT    NOT NULL,
     code       TEXT    NOT NULL,
     expires_at DATETIME NOT NULL,
     used       INTEGER NOT NULL DEFAULT 0
@@ -61,4 +62,4 @@ CREATE INDEX IF NOT EXISTS idx_slots_event ON slots(event_id);
 CREATE INDEX IF NOT EXISTS idx_claims_slot ON claims(slot_id);
 CREATE INDEX IF NOT EXISTS idx_rsvps_event ON rsvps(event_id);
 CREATE INDEX IF NOT EXISTS idx_sessions_user ON sessions(user_id);
-CREATE INDEX IF NOT EXISTS idx_verification_codes_phone ON verification_codes(phone);
+CREATE INDEX IF NOT EXISTS idx_verification_codes_identifier ON verification_codes(identifier);