Pretty URLs

Keep the current URL scheme, but add a slug generated from the title that is ignored when resolving the request.

Set the canonical URL to the one with the slug.

```
https://start.sch.bme.hu/posts/1
|
V
https://start.sch.bme.hu/posts/1-hello-world
```

https://developers.google.com/search/docs/crawling-indexing/canonicalization

Start by locating the post URL routing and the code that generates canonical URLs. Verify that both /posts/1 and /posts/1-hello-world resolve to the same post, while the canonical URL uses the title-derived slug; no specific files or tests are named in the issue.

Written by the indexing model from the issue text.