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.

units/fenris-collect.service

@@ -7,3 +7,4 @@ After=local-fs.target
 Type=oneshot
 ExecStart=/usr/libexec/fenris/fenris-collect
 TimeoutStartSec=90
+UMask=002