added permanent artifact links for latest green builds: /branches/<branch-key>/... serves the latest green build's artifacts (green-only, resolved per request), keepLatestGreen retention protection, permanent artifact-index page, and latestGreenUrl links in the branches view

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/main/kotlin/de/hoennig/gittally/config/GitTallyConfig.kt

@@ -38,6 +38,12 @@ data class BuildsConfig(
 
 data class ArtifactsConfig(
     val retentionPerBranch: Int = 3,
+    /**
+     * Keep each branch's latest green (SUCCESS) build beyond [retentionPerBranch],
+     * so the permanent `/branches/<branch-key>/…` artifact URLs stay valid while newer
+     * builds fail; the build is still dropped once its branch is gone from origin.
+     */
+    val keepLatestGreen: Boolean = true,
     /**
      * Root directory for stored build artifacts; empty means the platform default
      * `XDG_STATE_HOME` (or `~/.local/state`) + `/gittally/artifacts/<repo-key>`.