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/build/RunningBuild.kt

@@ -13,7 +13,11 @@ data class RunningBuild(
     val stagingDir: Path,
     /** Combined stdout+stderr log, written live while the build runs. */
     val liveLogFile: Path,
-)
+) {
+    /** Set by the executor when the build leaves the queue and starts executing. */
+    @Volatile
+    var runningSince: Instant? = null
+}
 
 /** Published via Spring's `ApplicationEventPublisher` on every persisted status transition. */
 data class BuildStatusChangedEvent(