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/StickinessSignal.php

@@ -8,10 +8,7 @@ final class StickinessSignal extends AbstractSignal
 {
     public function compute(SignalContext $context): array
     {
-        $isSqlite = DB::connection()->getDriverName() === 'sqlite';
-        $diffExpr = $isSqlite
-            ? 'CAST((julianday(MAX(price_effective_at)) - julianday(MIN(price_effective_at))) AS INTEGER)'
-            : 'DATEDIFF(MAX(price_effective_at), MIN(price_effective_at))';
+        $diffExpr = DbDialect::maxMinDayDiffExpr('price_effective_at');
 
         $rows = DB::table('station_prices')
             ->where('fuel_type', $context->fuelType->value)