feat(ci): Add Fedora RPM package build workflow

Build and publish RPM packages for Fedora using rpkg and official
rust-packaging macros. Packages are automatically signed by Forgejo's
built-in package registry (introduced in v9.0).

Publishes packages to organised groups:
- continuwuity (binary): base group (stable/dev/branch-name)
- continuwuity-debuginfo: GROUP-debug
- continuwuity (source RPM): 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 sanitised branch name versioning.

Uses dnf builddep to install build dependencies directly from the
generated SRPM, ensuring consistency between CI and spec file without
duplication. This also prevents hiding packaging issues that could
occur with --nodeps fallbacks.

pkg/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-# TODO: rpkg-util is no longer maintained, find a replacement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 }}}