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/DbDialect.php

@@ -0,0 +1,40 @@
+<?php
+
+namespace App\Services\Prediction\Signals;
+
+use Illuminate\Support\Facades\DB;
+
+/**
+ * SQL dialect helpers for the small set of MySQL/SQLite differences the
+ * signal classes care about. Centralises the isSqlite ternaries that were
+ * duplicated across DayOfWeekSignal and StickinessSignal.
+ */
+final class DbDialect
+{
+    private static function isSqlite(): bool
+    {
+        return DB::connection()->getDriverName() === 'sqlite';
+    }
+
+    /**
+     * Day-of-week expression returning 1=Sun..7=Sat (MySQL DAYOFWEEK convention).
+     * Targets a column on the queried table.
+     */
+    public static function dayOfWeekExpr(string $column): string
+    {
+        return self::isSqlite()
+            ? "(CAST(strftime('%w', {$column}) AS INTEGER) + 1)"
+            : "DAYOFWEEK({$column})";
+    }
+
+    /**
+     * Whole-day difference between MAX and MIN of a datetime column, suitable
+     * for use in an aggregate selectRaw.
+     */
+    public static function maxMinDayDiffExpr(string $column): string
+    {
+        return self::isSqlite()
+            ? "CAST((julianday(MAX({$column})) - julianday(MIN({$column}))) AS INTEGER)"
+            : "DATEDIFF(MAX({$column}), MIN({$column}))";
+    }
+}