refactor: modernise RPM builder with Python tooling v2

- Migrate from legacy shell-based build system to modern Python tooling
- Update all metadata.yaml files to new schema with per-distro builds
- Standardise build scripts with curl -L, envsubst, and error handling
- Convert nfpm.yaml templates to use environment variable substitution
- Update Dockerfile to accept all package metadata as build arguments
- Modernise Makefile to use new Python build tool commands
- Update CI workflow to use tools/build instead of make

rpms/nzbget_exporter/resources/build.sh

@@ -1,4 +1,6 @@
 #!/usr/bin/bash
+set -e
+
 cd /app
 git clone https://github.com/frebib/nzbget-exporter.git
 go install github.com/frebib/enumerx@latest
@@ -7,4 +9,8 @@ pushd /app/nzbget-exporter
 go generate
 go build -o ../nzbget_exporter
 popd
-nfpm pkg --config /app/resources/nfpm.yaml --target /app/dist --packager rpm
+
+# Process nfpm.yaml with envsubst
+envsubst < /app/resources/nfpm.yaml > /app/nfpm.yaml
+
+nfpm pkg --config /app/nfpm.yaml --target /app/dist --packager rpm