feat(sops): trigger service restarts on secret rotation

Wire up restartUnits on secrets whose consumers cache them in memory
(daemons read at startup), so sops-nix restarts the affected unit on
activation when the decrypted content changes:

- firefly: app_key → phpfpm-firefly-iii;
  auto_import_secret + access_token → phpfpm-firefly-iii-data-importer
- searx: secret_key → uwsgi
- opencode: web password → opencode-serve
- mail: sasl_passwd → postfix
- forgejo: gitea_dbpass → forgejo; runner-token → gitea-runner-default

Secrets read on demand by oneshots/timers (firefly sparda_pin, ntfy
token, restic backup creds, acme dns creds, wg conf) are left as-is.

systems/x86_64-linux/sgx/opencode.nix

@@ -42,5 +42,6 @@ in
   sops.secrets.opencode-web-password = {
     sopsFile = ../../../.secrets/sgx/opencode-web.yaml;
     owner = user;
+    restartUnits = [ "opencode-serve.service" ];
   };
 }