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/node_exporter/metadata.yaml

@@ -6,6 +6,7 @@ platform: linux
 maintainer: Prometheus
 homepage: https://github.com/prometheus/node_exporter
 license: Apache-2.0 license
+dist_tag: true
 builds:
 - repository:
   - almalinux/el8