chore: cut GitHub, make project Gitea-only

- relink README/CHANGELOG/pyproject/SECURITY to git.engelgardt23.ru
- port CI from GitHub Actions to .gitea/workflows (publishes Gitea release via API)
- rename GITHUB_REPO -> REPO; drop GitHub mentions in code/docs/comments
- remove .github (workflow + issue templates); drop made-by lines

SECURITY.md

@@ -6,16 +6,14 @@ on the host — so vulnerability reports are very welcome.
 
 ## Supported versions
 
-Only the latest tagged release on GitHub is supported. Older versions will not
+Only the latest tagged release is supported. Older versions will not
 get fixes; please upgrade first.
 
 ## How to report a vulnerability
 
 **Please do not open a public issue** for security-sensitive findings.
 
-Use GitHub's private security advisories: go to the
-[Security tab](../../security/advisories/new) of this repo and click
-"Report a vulnerability". GitHub will route it privately.
+Report privately by email to the maintainer at engelgardt2024@gmail.com.
 
 Please include:
 - The version you tested (the startup banner is enough).