Return a 404 when filtering the blog by a non-existent tag

[Amoifr]

Fixes #1454.

On the blog index, requesting a tag that does not exist (/blog/?tag=this-tag-does-not-exist) silently lists every latest post instead of telling the user the tag is unknown, because findOneBy() returns null and findLatest() then applies no tag filter.

As @GromNaN suggested on the issue, this returns a 404 when a tag is requested but not found, which is both more helpful for users and the expected behavior for SEO. Requesting an existing tag, or no tag at all, is unchanged.

Added two functional tests to BlogControllerTest: one for an existing tag (still 200) and one for an unknown tag (now 404).

Thanks @mindaugasvcs for the report and @GromNaN for the direction! 🙏

[GromNaN]

The initial ticket suggested removing this unnecessary DB query. The tag slug can be used to find articles directly; then return the 404 when no articles are found.

[Amoifr]

Good point, thanks! I reworked the PR accordingly: the Tag lookup is gone, findLatest() now filters directly on the tag name (through a dedicated join so posts keep all their tags), and the controller returns a 404 when no articles are found for the requested tag. The existing tests cover both cases and the full suite is green.