Fix team invitation email case sensitivity bug

- Add email normalization to TeamInvitation model using setEmailAttribute()
- Add HasFactory trait to Team model for testing support
- Create TeamFactory for testing
- Add tests to verify email normalization works correctly
- Fixes issue where mixed case emails in invitations would cause lookup failures
- Resolves #6291

The bug occurred because:
1. User model normalizes emails to lowercase
2. TeamInvitation model did not normalize emails
3. When invitation was created with mixed case, it was stored as-is
4. User lookup failed due to case mismatch during invitation acceptance
5. This caused users to not be able to see teams they were invited to

This fix ensures both models normalize emails consistently.

app/Models/Team.php

@@ -10,6 +10,7 @@ use App\Notifications\Channels\SendsSlack;
 use App\Traits\HasNotificationSettings;
 use App\Traits\HasSafeStringAttribute;
 use Illuminate\Database\Eloquent\Casts\Attribute;
+use Illuminate\Database\Eloquent\Factories\HasFactory;
 use Illuminate\Database\Eloquent\Model;
 use Illuminate\Notifications\Notifiable;
 use OpenApi\Attributes as OA;
@@ -37,7 +38,7 @@ use OpenApi\Attributes as OA;
 
 class Team extends Model implements SendsDiscord, SendsEmail, SendsPushover, SendsSlack
 {
-    use HasNotificationSettings, HasSafeStringAttribute, Notifiable;
+    use HasFactory, HasNotificationSettings, HasSafeStringAttribute, Notifiable;
 
     protected $guarded = [];