Record builds interrupted by a server shutdown as INTERRUPTED, not FAILED

A systemd stop killed the running build process and BuildExecutor
classified the death as FAILED, posting a red Gitea status; FAILED is
not restartable, so the startup recovery never re-enqueued the build.

A ContextClosedEvent listener now sets a shuttingDown flag, terminates
the process trees of executing builds, and drains until their
INTERRUPTED results are persisted. Queued builds stay PENDING without
starting a process; recovery re-enqueues both after the restart.
INTERRUPTED publishes as Gitea state "pending" instead of "failure",
since the build is going to be re-run.

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

src/test/kotlin/de/hoennig/gittally/gitea/GiteaClientTest.kt

@@ -121,10 +121,11 @@ class GiteaClientTest :
                     mapOf(
                         BuildStatus.SUCCESS to "success",
                         BuildStatus.FAILED to "failure",
-                        BuildStatus.INTERRUPTED to "failure",
                         BuildStatus.CANCELLED to "failure",
                         BuildStatus.PENDING to "pending",
                         BuildStatus.RUNNING to "pending",
+                        // interrupted builds are re-enqueued on startup, so the commit must not turn red
+                        BuildStatus.INTERRUPTED to "pending",
                     )
 
                 expectedStates.forEach { (status, state) ->