Fix F-04 + F-11; add AUDIT.md

- AUDIT.md: security audit report with 16 findings (F-01..F-16)
- F-04 (server.go): wire loginRateLimit (10 req/s, burst 10) to
  POST /v1/auth/login and POST /v1/token/validate; no limit on
  /v1/health or public-key endpoints
- F-04 (server_test.go): TestLoginRateLimited uses concurrent
  goroutines (sync.WaitGroup) to fire burst+1 requests before
  Argon2id completes, sidestepping token-bucket refill timing;
  TestTokenValidateRateLimited; TestHealthNotRateLimited
- F-11 (ui.go): refactor Register() so all UI routes are mounted
  on a child mux wrapped with securityHeaders middleware; five
  headers set on every response: Content-Security-Policy,
  X-Content-Type-Options, X-Frame-Options, HSTS, Referrer-Policy
- F-11 (ui_test.go): 7 new tests covering login page, dashboard
  redirect, root redirect, static assets, CSP directives,
  HSTS min-age, and middleware unit behaviour
Security: rate limiter on login prevents brute-force credential
  stuffing; security headers mitigate clickjacking (X-Frame-Options
  DENY), MIME sniffing (nosniff), and protocol downgrade (HSTS)

internal/db/accounts.go

@@ -127,7 +127,30 @@ func (db *DB) UpdatePasswordHash(accountID int64, hash string) error {
 	return nil
 }
 
+// StorePendingTOTP stores the encrypted TOTP secret without enabling the
+// totp_required flag. This is called during enrollment (POST /v1/auth/totp/enroll)
+// so the secret is available for confirmation without yet enforcing TOTP on login.
+// The flag is set to 1 only after the user successfully confirms the code via
+// handleTOTPConfirm, which calls SetTOTP.
+//
+// Security: keeping totp_required=0 during enrollment prevents the user from
+// being locked out if they abandon the enrollment flow after the secret is
+// generated but before they have set up their authenticator app.
+func (db *DB) StorePendingTOTP(accountID int64, secretEnc, secretNonce []byte) error {
+	_, err := db.sql.Exec(`
+		UPDATE accounts
+		SET totp_secret_enc = ?, totp_secret_nonce = ?, updated_at = ?
+		WHERE id = ?
+	`, secretEnc, secretNonce, now(), accountID)
+	if err != nil {
+		return fmt.Errorf("db: store pending TOTP: %w", err)
+	}
+	return nil
+}
+
 // SetTOTP stores the encrypted TOTP secret and marks TOTP as required.
+// Call this only after the user has confirmed the TOTP code; for the initial
+// enrollment step use StorePendingTOTP instead.
 func (db *DB) SetTOTP(accountID int64, secretEnc, secretNonce []byte) error {
 	_, err := db.sql.Exec(`
 		UPDATE accounts