feat(leads): link consumer account to lead on submission

- user_account FK on Lead model (SET_NULL, related_name='submitted_leads')
- Migration 0004_lead_user_account
- ScheduleCallView auto-matches consumer account by email on create
- _serialize_lead now returns userAccount: {id, name, email, phone, eventifyId, profilePicture}

admin_api/models.py

@@ -222,9 +222,13 @@ class Lead(models.Model):
     status      = models.CharField(max_length=20, choices=STATUS_CHOICES, default='new')
     source      = models.CharField(max_length=20, choices=SOURCE_CHOICES, default='schedule_call')
     priority    = models.CharField(max_length=10, choices=PRIORITY_CHOICES, default='medium')
-    assigned_to = models.ForeignKey(
+    assigned_to  = models.ForeignKey(
         User, on_delete=models.SET_NULL, null=True, blank=True, related_name='assigned_leads'
     )
+    user_account = models.ForeignKey(
+        User, on_delete=models.SET_NULL, null=True, blank=True, related_name='submitted_leads',
+        help_text='Consumer platform account that submitted this lead (auto-matched by email)'
+    )
     notes       = models.TextField(blank=True, default='')
     created_at  = models.DateTimeField(auto_now_add=True)
     updated_at  = models.DateTimeField(auto_now=True)