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/Support/Database.php

@@ -29,4 +29,14 @@ final class Database
     {
         return $this->pdo;
     }
+
+    /**
+     * SQL expression for the current UTC time, in the format every
+     * created_at/updated_at/*_at column uses (both as their DEFAULT in the
+     * migrations and wherever a repository sets one explicitly).
+     */
+    public static function nowExpr(): string
+    {
+        return "strftime('%Y-%m-%dT%H:%M:%SZ', 'now')";
+    }
 }