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>

rpm/mistralrs.spec

@@ -6,9 +6,16 @@
 %{!?mistralrs_version: %global mistralrs_version 0.7.0}
 %{!?mistralrs_flavour: %global mistralrs_flavour cuda13}
 
+# For prerelease builds, pass --define "mistralrs_prerelease 0.1.YYYYMMDDgitSHORTSHA"
+%if 0%{?mistralrs_prerelease:1}
+%global mistralrs_release %{mistralrs_prerelease}
+%else
+%global mistralrs_release 1
+%endif
+
 Name:           mistralrs-%{mistralrs_flavour}
 Version:        %{mistralrs_version}
-Release:        1%{?dist}
+Release:        %{mistralrs_release}%{?dist}
 Summary:        Fast, flexible LLM inference server (mistral.rs, %{mistralrs_flavour} flavour)
 
 License:        MIT