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 69 metadata.yaml files updated with dist_tag: true
- Exclude jellyfin from dist_tags

rpms/nomad/metadata.yaml

@@ -1,18 +1,21 @@
----
-arch: amd64
-builds:
-  - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
-    release: '1'
-    repository: [almalinux/el8]
-    version: 1.10.1
-  - image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
-    release: '1'
-    repository: [almalinux/el9]
-    version: 1.10.1
-description: A simple and flexible scheduler and orchestrator to deploy and manage containers and non-containerized applications across on-premises and clouds at scale.
+name: nomad
 github: unknown/nomad
+description: A simple and flexible scheduler and orchestrator to deploy and manage
+  containers and non-containerized applications across on-premises and clouds at scale.
+arch: amd64
+platform: linux
+maintainer: HashiCorp
 homepage: https://www.nomadproject.io/
 license: BUSL-1.1
-maintainer: HashiCorp
-name: nomad
-platform: linux
+dist_tag: true
+builds:
+- image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
+  release: '1'
+  repository:
+  - almalinux/el8
+  version: 1.10.1
+- image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+  release: '1'
+  repository:
+  - almalinux/el9
+  version: 1.10.1