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/Actions/Fortify/CreateNewUser.php

@@ -3,14 +3,14 @@
 namespace App\Actions\Fortify;
 
 use App\Concerns\PasswordValidationRules;
-use App\Concerns\ProfileValidationRules;
 use App\Models\User;
 use Illuminate\Support\Facades\Validator;
+use Illuminate\Validation\Rule;
 use Laravel\Fortify\Contracts\CreatesNewUsers;
 
 class CreateNewUser implements CreatesNewUsers
 {
-    use PasswordValidationRules, ProfileValidationRules;
+    use PasswordValidationRules;
 
     /**
      * Validate and create a newly registered user.
@@ -20,7 +20,8 @@ class CreateNewUser implements CreatesNewUsers
     public function create(array $input): User
     {
         Validator::make($input, [
-            ...$this->profileRules(),
+            'name' => ['required', 'string', 'max:255'],
+            'email' => ['required', 'string', 'email', 'max:255', Rule::unique(User::class)],
             'password' => $this->passwordRules(),
         ])->validate();