refactor: extract DbDialect helper, inline ProfileValidationRules trait

Audit items #17 and #21.

#17 — DayOfWeekSignal and StickinessSignal each had their own
isSqlite ternary picking between SQLite (strftime/julianday) and
MySQL (DAYOFWEEK/DATEDIFF) date expressions. Centralised in
App\Services\Prediction\Signals\DbDialect.

#21 — ProfileValidationRules was a trait with one consumer
(CreateNewUser); inlined the rules into the action and deleted the
trait. Also dropped PasswordValidationRules::currentPasswordRules()
which was unused. PasswordValidationRules trait stays (two consumers).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/Concerns/PasswordValidationRules.php

@@ -16,14 +16,4 @@ trait PasswordValidationRules
     {
         return ['required', 'string', Password::default(), 'confirmed'];
     }
-
-    /**
-     * Get the validation rules used to validate the current password.
-     *
-     * @return array<int, Rule|array<mixed>|string>
-     */
-    protected function currentPasswordRules(): array
-    {
-        return ['required', 'string', 'current_password'];
-    }
 }