Rotate DataHub internal authentication secrets

Generates fresh values for DATAHUB_SECRET (Play session signing key) and DATAHUB_SYSTEM_CLIENT_SECRET (system OAuth client secret), updates every DataHub app child accordingly, and triggers a coordinated restart so the new values take effect everywhere simultaneously. USER-VISIBLE SIDE EFFECT: every active browser session is invalidated -- all currently logged-in users will be signed out and must re-login after the coordinated restart. Inter-service calls may briefly return 401 errors while the rotation is in flight. DATAHUB_SYSTEM_CLIENT_ID is the fixed identifier "__datahub_system" and is intentionally left untouched.