fix(store): degrade on store permission errors, keep store group-readable (issue #54)

- open_store_readonly(): stat() PermissionError (non-group user on the
  2750 store dir) now maps to StoreFault so status/TUI degrade instead
  of crashing with a traceback.
- init_store(): chmod db + -wal/-shm group rw after WAL setup — SQLite
  WAL readers need write access to sidecars even for mode=ro opens.
- Store dir 2750 → 2770 (tmpfiles + make install) and UMask=002 on the
  collect unit so root-created files stay group-accessible.
- rpm %post upgrade path re-runs systemd-tmpfiles --create to correct
  placement modes on existing machines.
Bump to 0.3.3.

packaging/tmpfiles.d/fenris.conf

@@ -1,2 +1,2 @@
 # Type  Path              Mode  User   Group  Age  Argument
-d       /var/lib/fenris   2750  root   fenris  -   -
+d       /var/lib/fenris   2770  root   fenris  -   -