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/jellyfin-ffmpeg-bin/metadata.yaml

@@ -6,7 +6,7 @@ platform: linux
 maintainer: unkin
 homepage: https://github.com/jellyfin/jellyfin-ffmpeg
 license: GPL-3.0
-dist_tag: true
+dist_tag: false
 builds:
 - repository:
   - almalinux/el8

rpms/jellyfin-server/metadata.yaml

@@ -1,7 +1,7 @@
 name: jellyfin-server
 github: unknown/jellyfin-server
 description: jellyfin-server package
-dist_tag: true
+dist_tag: false
 builds:
 - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
   release: '1'

rpms/jellyfin-web/metadata.yaml

@@ -1,7 +1,7 @@
 name: jellyfin-web
 github: unknown/jellyfin-web
 description: jellyfin-web package
-dist_tag: true
+dist_tag: false
 builds:
 - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
   release: '1'