feat: add GymSession model and TrainingService

Implements gym training logic with daily session tracking:
- GymSession model manages training sessions per stat per day
- TrainingService handles training validation, energy consumption, and stat gains
- Lazy daily reset: session count resets when reset_date is before today
- Validates: sufficient energy, daily cap (5 sessions per stat), valid stats

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/Models/GymSession.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+
+/**
+ * @property int $id
+ * @property int $character_id
+ * @property string $stat
+ * @property int $sessions_today
+ * @property \Illuminate\Support\Carbon $reset_date
+ */
+class GymSession extends Model
+{
+    protected $fillable = [
+        'character_id',
+        'stat',
+        'sessions_today',
+        'reset_date',
+    ];
+
+    protected $casts = [
+        'reset_date' => 'date',
+    ];
+
+    public function character(): BelongsTo
+    {
+        return $this->belongsTo(Character::class);
+    }
+}