Centralize the current-timestamp SQL expression

"strftime('%Y-%m-%dT%H:%M:%SZ', 'now')" was a private nowExpr()
method copy-pasted identically in CardRepository, CardStatusRepository
and ProjectRepository, and inlined as a raw literal directly in
PasskeyRepository, WebAuthnChallengeRepository, UserRepository and
EmailVerificationRepository -- 7 files, ~12 occurrences of the same
string. Now one Database::nowExpr() static method (Support/Database
already being the natural home for SQLite-specific concerns), used
everywhere a repository sets a timestamp explicitly.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/Repository/UserRepository.php

@@ -4,6 +4,7 @@ declare(strict_types=1);
 
 namespace App\Repository;
 
+use App\Support\Database;
 use PDO;
 use PDOException;
 
@@ -88,10 +89,9 @@ final class UserRepository
 
     public function markEmailVerified(int $id): void
     {
+        $now = Database::nowExpr();
         $this->pdo->prepare(
-            "UPDATE users SET email_verified_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now'),
-                    updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
-             WHERE id = :id"
+            "UPDATE users SET email_verified_at = {$now}, updated_at = {$now} WHERE id = :id"
         )->execute(['id' => $id]);
     }
 
@@ -100,18 +100,16 @@ final class UserRepository
      */
     public function updateEmail(int $id, string $email): void
     {
+        $now = Database::nowExpr();
         $this->pdo->prepare(
-            "UPDATE users SET email = :email,
-                    email_verified_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now'),
-                    updated_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now')
-             WHERE id = :id"
+            "UPDATE users SET email = :email, email_verified_at = {$now}, updated_at = {$now} WHERE id = :id"
         )->execute(['email' => $email, 'id' => $id]);
     }
 
     public function markVerificationEmailSent(int $id): void
     {
         $this->pdo->prepare(
-            "UPDATE users SET verification_email_sent_at = strftime('%Y-%m-%dT%H:%M:%SZ', 'now') WHERE id = :id"
+            'UPDATE users SET verification_email_sent_at = ' . Database::nowExpr() . ' WHERE id = :id'
         )->execute(['id' => $id]);
     }