feat(ci): Add Fedora RPM package build workflow

Build and publish RPM packages for Fedora using rpkg and official
rust-packaging macros. GPG sign packages with Ed25519 repository key
and deploy to Forgejo package registry.

Package organisation works around Forgejo's one-file-per-version limit:
- Binary packages: base group (stable/dev/branch-name)
- Debug packages: GROUP-debug
- Source packages: GROUP-src

Workflow triggers on pushes to relevant paths and version tags (v*).
Tagged releases use clean version numbers (v1.2.3 becomes 1.2.3-1)
while branch builds use git SHA versioning.

Include GPG public key for package verification and documentation
for RPM repository configuration and installation methods.

fedora/continuwuity.spec.rpkg

@@ -1,6 +1,5 @@
-# This should be run using rpkg-util: https://docs.pagure.org/rpkg-util
+# This should be run using rpkg: https://docs.pagure.org/rpkg
 # it requires Internet access and is not suitable for Fedora main repos
-# TODO: rpkg-util is no longer maintained, find a replacement
 
 Name:           continuwuity
 Version:        {{{ git_repo_version }}}