Separate queue wait from build duration

BuildResult gains runningSince, set when a build leaves the queue; the
recorded duration now measures pure build time from that point, so build
runtimes can be tracked without queue wait. A build cancelled while still
queued records neither. The UI shows the live wait time in italics while
pending and switches to the real build time once the build runs; the Gitea
"after mm:ss" descriptions now also report pure build time.

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

src/main/kotlin/de/hoennig/gittally/server/BuildsApiController.kt

@@ -60,6 +60,7 @@ class BuildsApiController(
                     (results.firstOrNull { it.artifactKey == build.artifactKey }?.status ?: BuildStatus.RUNNING)
                         .jsonName,
                 startedAt = build.startedAt,
+                runningSince = build.runningSince,
                 logSize = liveLogSize(build.liveLogFile),
             )
         }