feat: dist_tag boolean for distro-aware release strings

Adds a per-package dist_tag: true/false metadata flag (default false).
When enabled the build tool appends the RPM dist tag to the release
at build time so each distro produces a unique version in the Gitea
package registry:

  release: 1  +  almalinux/el9  ->  PACKAGE_RELEASE=1.el9
  release: 1  +  fedora/43      ->  PACKAGE_RELEASE=1.fc43

This ensures Gitea package existence checks don't confuse packages built
for one distro with those built for another (the original bug).

Changes:
- Add effective_release() and get_rpm_dist_tag() helpers
- Revert the broken files-endpoint check from the previous commit;
  dist disambiguation is now handled purely via the release string
- discover_packages and build_single both compute the effective release
  before constructing PackageInfo
- check_package_exists simplified back to a single version-level check
- dist_tag added to METADATA_SCHEMA and PackageMetadata dataclass
- All 72 metadata.yaml files updated with dist_tag: true

rpms/jsonnet-language-server/metadata.yaml

@@ -1,4 +1,13 @@
+name: jsonnet-language-server
+github: grafana/jsonnet-language-server
+description: Jsonnet Language Server Protocol implementation for the Jsonnet templating
+  language.
 arch: amd64
+platform: linux
+maintainer: Grafana Labs
+homepage: https://github.com/grafana/jsonnet-language-server
+license: Apache-2.0
+dist_tag: true
 builds:
 - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
   release: 1
@@ -10,11 +19,3 @@ builds:
   repository:
   - almalinux/el9
   version: 0.17.0
-description: Jsonnet Language Server Protocol implementation for the Jsonnet templating
-  language.
-github: grafana/jsonnet-language-server
-homepage: https://github.com/grafana/jsonnet-language-server
-license: Apache-2.0
-maintainer: Grafana Labs
-name: jsonnet-language-server
-platform: linux