feat: add all Eloquent models with relationships

app/Models/Activity.php

@@ -0,0 +1,38 @@
+<?php
+
+namespace App\Models;
+
+use Illuminate\Database\Eloquent\Model;
+use Illuminate\Database\Eloquent\Relations\BelongsTo;
+use Illuminate\Database\Eloquent\Relations\BelongsToMany;
+
+class Activity extends Model
+{
+    protected $fillable = [
+        'title', 'description', 'start_date', 'end_date',
+        'created_by', 'status', 'approved_by', 'approved_at',
+        'executed_at', 'execution_notes',
+    ];
+
+    protected $casts = [
+        'start_date'  => 'date',
+        'end_date'    => 'date',
+        'approved_at' => 'datetime',
+        'executed_at' => 'datetime',
+    ];
+
+    public function creator(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'created_by');
+    }
+
+    public function approver(): BelongsTo
+    {
+        return $this->belongsTo(User::class, 'approved_by');
+    }
+
+    public function participants(): BelongsToMany
+    {
+        return $this->belongsToMany(User::class, 'activity_member');
+    }
+}