fix(ha-commit-engine): raise commit-barrier default 50->500ms + add --sync-timeout-ms

Live symptom: jobsub intermittently failed with 'Connection timed out' even
though the cluster had quorum. Root cause: the synchronous commit barrier waits
only DEFAULT_SYNC_TIMEOUT_MS=50ms for durable majority replication, and that was
not configurable. 50ms is too tight for durable sync majority replication —
especially since standbys now fdatasync each replicated entry before ACK — so a
multi-entry jobsub's later WAL entry intermittently misses the window:
  commit: WAIT deadline expired without commit (lsn=3 committed_lsn=2 timeout_ms=50)
-> persist hook returns -ETIMEDOUT -> handle_jobsubmit rejects -> client ETIMEDOUT.

Fix:
- Raise DEFAULT_SYNC_TIMEOUT_MS 50 -> 500 (config.h) — realistic for durable sync
  majority replication, still bounded for failure detection.
- Add a --sync-timeout-ms controller flag threaded arguments -> myconf ->
  persist pcfg (override applied before persist_init); 0 = use default, positive
  overrides — so operators can tune per environment.

TDD: literal-pinned persist_config_defaults().sync_timeout_ms == 500. All
timeout-sensitive tests already pass an explicit timeout_ms (none rely on the
default), so none are slowed; full controller suite green under ASAN. No
commit-engine/barrier/replication/wire change beyond the default value + plumbing.
bug-561.

src/controller/testsuite/test_cluster_persist.c

@@ -60,6 +60,11 @@ static void test_persist_config_defaults_finished_retain_s(void)
 static void test_persist_config_defaults_sync_timeout_ms(void)
 {
     persist_config_t cfg = persist_config_defaults();
+    /* Default raised to 500 ms: realistic for durable sync majority
+     * replication (each standby fdatasyncs before ACK), still bounded for
+     * failure detection. Literal-pinned so a regression to the old 50 ms
+     * tight window fails here, not just on a live cluster. */
+    TEST_ASSERT_EQUAL_UINT32(500, cfg.sync_timeout_ms);
     TEST_ASSERT_EQUAL_UINT32(DEFAULT_SYNC_TIMEOUT_MS, cfg.sync_timeout_ms);
 }