romm: Let the database container own its data directory

tmpfiles re-applied root:root to /var/lib/romm/db on every switch. A
MariaDB running as uid 999 then could not delete tc.log on shutdown,
so the next start took it for a crash and failed recovery with "Bad
magic header in tc log".

hosts/manatee/modules/romm/default.nix

@@ -55,7 +55,11 @@ in
 
     systemd.tmpfiles.rules = [
       "d /var/lib/romm 0755 root root -"
-      "d /var/lib/romm/db 0755 root root -"
+      # Owned by the container's mysql user (999). tmpfiles re-applies owner
+      # and mode on every switch; as root:root it left a running MariaDB unable
+      # to delete tc.log on shutdown, so the next start failed "crash recovery"
+      # with "Bad magic header in tc log".
+      "d /var/lib/romm/db 0755 999 999 -"
       "d /var/lib/romm/redis 0755 999 1000 -"
       "d /var/lib/romm/resources 0755 root root -"
       "d /var/lib/romm/assets 0755 root root -"