fix(ci): give the backend test fork 120s to shut down

All 2327 tests pass, but the build went red: after the suite finishes,
Surefire calls System.exit(0) and the single reused fork then closes ~32
cached Spring contexts at once — each tearing down a Testcontainers Postgres
+ HikariCP pool — which overruns Surefire's 30s default post-exit grace.
Surefire force-kills the fork and reports a fork timeout (BUILD FAILURE with
0 failures). The session-cleanup InterruptedException and Testcontainers
reaper NotFoundException in the log are symptoms of that contended teardown.

Set the previously-unset forkedProcessExitTimeoutInSeconds to 120s. This is a
different knob from forkedProcessTimeoutInSeconds (total/inactivity), already
600s, which is why the earlier ceiling bumps never addressed this failure.

Phase B of #848; the durable fix (singleton Testcontainers Postgres +
disabling the Spring Session JDBC cleanup scheduler in tests) follows.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/pom.xml

@@ -369,6 +369,12 @@
 				<artifactId>maven-surefire-plugin</artifactId>
 				<configuration>
 					<forkedProcessTimeoutInSeconds>600</forkedProcessTimeoutInSeconds>
+					<!-- Grace period after the test JVM calls System.exit(0). The 30s default is too
+					     short: the single reused fork closes ~32 cached Spring contexts at shutdown,
+					     each tearing down a Testcontainers Postgres + HikariCP pool, which overruns 30s
+					     and makes Surefire kill the fork (BUILD FAILURE despite 0 test failures). This is
+					     a different knob from forkedProcessTimeoutInSeconds above. See issue #848. -->
+					<forkedProcessExitTimeoutInSeconds>120</forkedProcessExitTimeoutInSeconds>
 					<systemPropertyVariables>
 						<junit.jupiter.execution.timeout.default>90 s</junit.jupiter.execution.timeout.default>
 					</systemPropertyVariables>