Bug: Post title publicly visible, but content forbidden

## Bug Report

**Current Behavior / Steps to Reproduce**
- Create one tag that is publicly visible and one that isn't.
- Create a post with both tags.
- Log out
- The title is visible on the tag overview but when clicked the content is not found

**Expected Behavior**
I hoped I could create a post that is publicly visible but also marked with a private tag.

**Screenshots**
x

**Environment**
- Flarum version: 1.2.0
- Webserver: nginx in docker
- Hosting environment: docker on VPS
- PHP version: 8.0
- Browser: Firefox 96.0.3

```
Flarum core 1.2.0
PHP version: 8.0.14
MySQL version: 5.5.5-10.5.1-MariaDB-1:10.5.1+maria~bionic
Loaded extensions: Core, phpdbg_webhelper, date, libxml, pcre, zlib, filter, hash, json, readline, Reflection, SPL, session, ctype, curl, dom, fileinfo, gd, gmp, iconv, intl, mbstring, openssl, PDO, standard, tokenizer, xmlwriter, zip, exif, mysqlnd, Phar, pdo_mysql, apcu, Zend OPcache
+---------------------------+---------+--------+
| Flarum Extensions | | |
+---------------------------+---------+--------+
| ID | Version | Commit |
+---------------------------+---------+--------+
| flarum-nicknames | v1.2.0 | |
| flarum-flags | v1.2.0 | |
| spookygames-auth-keycloak | 1.0.0.5 | |
| fof-upload | 1.1.0 | |
| fof-links | 1.0.3 | |
| flarum-tags | v1.2.0 | |
| flarum-suspend | v1.2.0 | |
| flarum-subscriptions | v1.2.0 | |
| flarum-sticky | v1.2.0 | |
| flarum-statistics | v1.2.0 | |
| flarum-mentions | v1.2.0 | |
| flarum-markdown | v1.2.0 | |
| flarum-lock | v1.2.0 | |
| flarum-likes | v1.2.0 | |
| flarum-lang-german | 1.2.1 | |
| flarum-lang-english | v1.2.0 | |
| flarum-emoji | v1.2.0 | |
| flarum-bbcode | v1.2.0 | |
| flarum-approval | v1.2.0 | |
+---------------------------+---------+--------+
Base URL: x
Installation path: /flarum/app
Queue driver: sync
Mail driver: smtp
Debug mode: ON
```

**Possible Solution**
The post should be publicly visible because one tag is public.

Start by reproducing the case with one public tag and one private tag on the same post, then trace the tag overview and post-loading authorization paths. Confirm whether visibility is evaluated per tag or across all tags, and add or update coverage for the expected logged-out behavior before verifying the result through the tag overview.

Written by the indexing model from the issue text.