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/unrar/metadata.yaml

@@ -1,18 +1,22 @@
----
-arch: amd64
-builds:
-  - image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
-    release: '1'
-    repository: [almalinux/el8]
-    version: '7.20'
-  - image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
-    release: '1'
-    repository: [almalinux/el9]
-    version: '7.20'
-description: WinRAR is a powerful archive manager. It can backup your data and reduce the size of email attachments, open and unpack RAR, ZIP and other files downloaded from Internet, create new archives in RAR and ZIP file format.
+name: unrar
 github: unknown/unrar
+description: WinRAR is a powerful archive manager. It can backup your data and reduce
+  the size of email attachments, open and unpack RAR, ZIP and other files downloaded
+  from Internet, create new archives in RAR and ZIP file format.
+arch: amd64
+platform: linux
+maintainer: RARLAB
 homepage: https://www.rarlab.com/
 license: Freeware (UNRAR 7.11 freeware Copyright (c) 1993-2025 Alexander Roshal)
-maintainer: RARLAB
-name: unrar
-platform: linux
+dist_tag: true
+builds:
+- image: git.unkin.net/unkin/almalinux8-rpmbuilder:latest
+  release: '1'
+  repository:
+  - almalinux/el8
+  version: '7.20'
+- image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
+  release: '1'
+  repository:
+  - almalinux/el9
+  version: '7.20'