Notify question authors via Resend when an admin posts the first answer.

Require email at registration (editable on profile), add a Resend mailer with idempotent first-answer sends, and keep answer saves independent of delivery.

schema.sql

@@ -4,11 +4,16 @@ CREATE TABLE IF NOT EXISTS users (
   name TEXT NOT NULL,
   password_hash TEXT NOT NULL,
   role TEXT NOT NULL DEFAULT 'user' CHECK (role IN ('user', 'admin')),
+  email TEXT NOT NULL DEFAULT '',
   avatar_url TEXT NOT NULL DEFAULT '',
   state TEXT NOT NULL DEFAULT '',
   created_at TEXT NOT NULL
 );
 
+CREATE UNIQUE INDEX IF NOT EXISTS users_email_lower_uidx
+  ON users (lower(email))
+  WHERE email <> '';
+
 CREATE TABLE IF NOT EXISTS questions (
   id TEXT PRIMARY KEY,
   author_id TEXT NOT NULL REFERENCES users(id),