Publish vendor RPMs to artifactapi rpm-vendor, one package per job

Builds currently push to Gitea's RPM registry and run build-all in a single
job. Publish instead to the artifactapi rpm-vendor local repos (real yum
repos; repodata regenerates automatically) and give each vendor package its
own job so failures are isolated and builds parallelise. Because builds no
longer land in Gitea, the "skip already-built version" check now probes
artifactapi.

- tools/build: repoint check_package_exists from the Gitea packages API to
  the artifactapi rpm-vendor repo (GET the served Packages/<file>.rpm path).
  Add get_vendor_repo() (distro -> rpm-vendor-<suffix>) and get_rpm_arch()
  (amd64 -> x86_64), and carry arch on PackageInfo so the probed filename
  matches the built RPM.
- .woodpecker: replace the per-distro build-all pipelines with per-distro
  matrix pipelines over PACKAGE, so each package builds in its own job.
  deploy-*.yaml (master push) build each package and PUT its RPM to the
  distro's rpm-vendor repo; build-*.yaml (pull_request) build as a check.
- tools/gen-pipelines + `make pipelines`: generate the .woodpecker matrix
  pipelines from rpms/*/metadata.yaml (source of truth for target distros);
  regenerate after adding or removing packages.

.woodpecker/build-fedora42.yaml

@@ -1,13 +1,25 @@
+# Generated by tools/gen-pipelines - do not edit by hand.
 when:
   - event: pull_request
 
+matrix:
+  PACKAGE:
+    - consul
+    - nfpm
+    - tea
+    - unkin-ca-certificates
+    - uv
+    - vault
+    - xh
+    - yq
+
 steps:
-  - name: build rpms
+  - name: build
     image: git.unkin.net/unkin/almalinux9-rpmbuilder:latest
     commands:
       - mkdir -p /woodpecker/rpms
       - ln -s /woodpecker/rpms /workspace
-      - ./tools/build build-all --distro fedora/42 --buildah
+      - ./tools/build build --distro fedora/42 --buildah "$PACKAGE"
     privileged: true
     backend_options:
       kubernetes:
@@ -19,8 +31,3 @@ steps:
           limits:
             memory: 2Gi
             cpu: 2
-
-  - name: show rpms
-    image: git.unkin.net/unkin/fedora42-base:latest
-    commands:
-      - find /woodpecker/src/git.unkin.net/unkin/rpmbuilder/ -type f -name "*.rpm"