feat: add prerelease RPM builds from upstream main branch

Poll upstream main branch HEAD alongside release tags. When a new commit
is detected, build and publish prerelease RPMs to a separate unstable
repo at rpm.lair.cafe/fedora/$releasever/$basearch/unstable/.

RPM versioning uses the Fedora snapshot convention (e.g.
0.8.1-0.1.20260511git1a2b3c4.fc43) so stable releases automatically
supersede any installed prerelease.

- RPM spec: conditional Release field via mistralrs_prerelease define
- poll-upstream.yml: new check-prerelease job fetches main HEAD + Cargo.toml version
- build-prerelease.yml: new workflow for commit-based builds without --locked
- UI: fetch both stable/unstable manifests, show channel badges, add
  unstable repo setup instructions to home page

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

.gitea/workflows/poll-upstream.yml

@@ -83,3 +83,73 @@ jobs:
             --header 'Content-Type: application/json' \
             --url "${{ github.server_url }}/api/v1/repos/${{ github.repository }}/actions/workflows/build-release.yml/dispatches" \
             --data "{\"ref\":\"refs/heads/main\",\"inputs\":{\"tag\":\"${{ steps.upstream.outputs.tag }}\"}}"
+
+  check-prerelease:
+    runs-on: fedora-43
+    steps:
+      - name: Get upstream main branch HEAD
+        id: upstream
+        run: |
+          response=$(curl --silent --show-error --fail --location \
+            --header 'Accept: application/vnd.github+json' \
+            --url 'https://api.github.com/repos/EricLBuehler/mistral.rs/commits/main')
+          sha=$(echo "${response}" | jq -r .sha)
+          short_sha=$(echo "${sha}" | head --bytes=7)
+          date=$(echo "${response}" | jq -r '.commit.committer.date[:10]' | tr -d '-')
+          echo "sha=${sha}" >> "$GITHUB_OUTPUT"
+          echo "short_sha=${short_sha}" >> "$GITHUB_OUTPUT"
+          echo "date=${date}" >> "$GITHUB_OUTPUT"
+          echo "Upstream main HEAD: ${sha} (${date})"
+
+      - name: Get version from upstream Cargo.toml
+        id: version
+        run: |
+          version=$(curl --silent --show-error --fail --location \
+            --header 'Accept: application/vnd.github.raw+json' \
+            --url "https://api.github.com/repos/EricLBuehler/mistral.rs/contents/Cargo.toml?ref=${{ steps.upstream.outputs.sha }}" \
+            | grep '^version' | head --lines=1 | sed 's/.*"\(.*\)".*/\1/')
+          echo "version=${version}" >> "$GITHUB_OUTPUT"
+          echo "Upstream Cargo.toml version: ${version}"
+
+      - name: Check if prerelease is already published
+        id: published
+        run: |
+          prerelease="0.1.${UPSTREAM_DATE}git${UPSTREAM_SHORT_SHA}"
+          needs_build=false
+          for target in "43:cuda13"; do
+            fedora_version="${target%%:*}"
+            flavour="${target##*:}"
+            base_url="https://rpm.lair.cafe/fedora/${fedora_version}/x86_64/unstable"
+            rpm_name="mistralrs-${flavour}-${UPSTREAM_VERSION}-${prerelease}.fc${fedora_version}.x86_64.rpm"
+
+            http_code=$(curl \
+                --silent \
+                --write-out "%{http_code}" \
+                --output /dev/null \
+                --head \
+                --url "${base_url}/${rpm_name}")
+            if [ "${http_code}" = "404" ]; then
+              echo "missing: ${base_url}/${rpm_name}"
+              needs_build=true
+              continue
+            elif [ "${http_code}" != "200" ]; then
+              echo "unexpected HTTP ${http_code} for ${base_url}/${rpm_name}"
+              exit 1
+            fi
+            echo "found: ${base_url}/${rpm_name}"
+          done
+          echo "already_built=$( [ "${needs_build}" = "true" ] && echo false || echo true )" >> "$GITHUB_OUTPUT"
+        env:
+          UPSTREAM_VERSION: ${{ steps.version.outputs.version }}
+          UPSTREAM_DATE: ${{ steps.upstream.outputs.date }}
+          UPSTREAM_SHORT_SHA: ${{ steps.upstream.outputs.short_sha }}
+
+      - name: Trigger prerelease build workflow
+        if: steps.published.outputs.already_built == 'false'
+        run: |
+          curl --fail --silent --show-error --location \
+            --request POST \
+            --header "Authorization: token ${{ secrets.DISPATCH_TOKEN }}" \
+            --header 'Content-Type: application/json' \
+            --url "${{ github.server_url }}/api/v1/repos/${{ github.repository }}/actions/workflows/build-prerelease.yml/dispatches" \
+            --data "{\"ref\":\"refs/heads/main\",\"inputs\":{\"commit\":\"${{ steps.upstream.outputs.sha }}\",\"version\":\"${{ steps.version.outputs.version }}\",\"date\":\"${{ steps.upstream.outputs.date }}\",\"short_sha\":\"${{ steps.upstream.outputs.short_sha }}\"}}"