fix: keep the shutdown drain inside Kubernetes' grace period, document 0 as auto

Both from dkrizan's review:

- awaitTerminationSeconds was 20 per pool, and Spring destroys pools one at a time,
  so four pools could wait up to 80s. The Tolgee chart does not set
  terminationGracePeriodSeconds, so pods get Kubernetes' default of 30 and the
  kubelet would SIGKILL mid-drain — the wait would cost the whole grace period and
  still lose the work it was protecting. 5s per pool keeps the worst case at 20s.
- max-threads treats 0 as "derive it", so someone setting 0 to switch a pool off
  gets the derived size instead. The property docs now say so for both pools.

Author: JanCizmar

backend/app/src/main/kotlin/io/tolgee/configuration/AsyncExecutorFactory.kt

@@ -83,7 +83,13 @@ class AsyncExecutorFactory(
 
     /** A burst is a burst regardless of how many threads drain it; 3 threads still deserve a buffer. */
     const val MIN_QUEUE_CAPACITY = 50
-    const val SHUTDOWN_DRAIN_SECONDS = 20
+
+    /**
+     * Spring destroys the pools one at a time, so this is paid once per pool. Four pools at 5s stay
+     * inside Kubernetes' default terminationGracePeriodSeconds of 30, which the Tolgee chart does
+     * not override — going over it means the kubelet SIGKILLs mid-drain and the wait buys nothing.
+     */
+    const val SHUTDOWN_DRAIN_SECONDS = 5
 
     const val UNBOUNDED_QUEUE = Int.MAX_VALUE
 

backend/data/src/main/kotlin/io/tolgee/configuration/tolgee/AsyncProperties.kt

@@ -34,7 +34,9 @@ class StreamingAsyncProperties {
     description =
       "How many streaming responses this instance can produce at the same time.\n\n" +
         "Each one occupies one thread **and one database connection** until the response is " +
-        "finished, so this must stay well below your database connection pool size.",
+        "finished, so this must stay well below your database connection pool size.\n\n" +
+        "There is no way to switch streaming off: `0` and any negative value mean *derive it*, " +
+        "not *disable it*.",
     defaultValue = "-1",
     defaultExplanation = "A third of the database connection pool size, never less than 2",
   )
@@ -72,7 +74,10 @@ class StreamingAsyncProperties {
 )
 class BackgroundAsyncProperties {
   @DocProperty(
-    description = "How many background tasks Tolgee runs in parallel on this instance.",
+    description =
+      "How many background tasks Tolgee runs in parallel on this instance.\n\n" +
+        "There is no way to switch background processing off: `0` and any negative value mean " +
+        "*derive it*, not *disable it*.",
     defaultValue = "-1",
     defaultExplanation = "A sixth of the database connection pool size, never less than 2",
   )