Add SPDX headers

[AndyScherzinger]

• and GH check
• fix license identifier to reflect the "or-later" aspect given GPL-3.0 isn't a valid identifier. Needs to be -only or -or-later
• added a reuse status badge

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I expect the -or-later is in line with you @ChristophWurst given all other libs are also GPL-3.0-or-later

[AndyScherzinger]

Also a release would be highly appreciated, to help with getting the server repo to REUSE compliance :)
So this is one of two items that need publishing :D