Add 6-digit login code for mobile authentication

Magic links open in the device browser, not the OAuth in-app browser,
so the session doesn't carry over. A login code lets mobile users
type a 6-digit code from their email into the login page instead.

- Generate 6-digit numeric code alongside magic link token
- Store code in magic_tokens table
- Add verify-code step to login API endpoint
- Show code input UI after magic link is sent
- Include code in email template
- i18n keys for code UI (en + de)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/db/src/schema/journal.ts

@@ -49,6 +49,7 @@ export const magicTokens = journalSchema.table("magic_tokens", {
   id: text("id").primaryKey(),
   email: text("email").notNull(),
   token: text("token").notNull().unique(),
+  code: text("code"),
   purpose: text("purpose").notNull().default("login"),
   expiresAt: timestamp("expires_at", { withTimezone: true }).notNull(),
   usedAt: timestamp("used_at", { withTimezone: true }),