Watching: Prevent issues when watchable or user is deleted

- Adds filtering to the watched items list in notification preferences
  so that deleted (recycle bin) items are removed via query.
- Adds relations and logic to properly remove watches upon user and
  entity delete events, to old watches in database do not linger.
- Adds testing to cover the above.

Did not add migration for existing data, since patch will be close to
introduction, and lingering DB entries don't open a security concern,
just some potential confusion in specific potential scenarios.
Probably not work extra migration risk, although could add in future if
concerns/issues are found.

Related to #4499

app/Users/UserRepo.php

@@ -18,18 +18,13 @@ use Illuminate\Support\Str;
 
 class UserRepo
 {
-    protected UserAvatars $userAvatar;
-    protected UserInviteService $inviteService;
-
-    /**
-     * UserRepo constructor.
-     */
-    public function __construct(UserAvatars $userAvatar, UserInviteService $inviteService)
-    {
-        $this->userAvatar = $userAvatar;
-        $this->inviteService = $inviteService;
+    public function __construct(
+        protected UserAvatars $userAvatar,
+        protected UserInviteService $inviteService
+    ) {
     }
 
+
     /**
      * Get a user by their email address.
      */
@@ -155,6 +150,7 @@ class UserRepo
         $user->apiTokens()->delete();
         $user->favourites()->delete();
         $user->mfaValues()->delete();
+        $user->watches()->delete();
         $user->delete();
 
         // Delete user profile images