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/Services/Prediction/Signals/DayOfWeekSignal.php

@@ -10,10 +10,7 @@ final class DayOfWeekSignal extends AbstractSignal
 
     public function compute(SignalContext $context): array
     {
-        $isSqlite = DB::connection()->getDriverName() === 'sqlite';
-        $dowExpr = $isSqlite
-            ? "(CAST(strftime('%w', price_effective_at) AS INTEGER) + 1)"
-            : 'DAYOFWEEK(price_effective_at)';
+        $dowExpr = DbDialect::dayOfWeekExpr('price_effective_at');
 
         $rows = DB::table('station_prices')
             ->where('fuel_type', $context->fuelType->value)