feat: add backup module with Restic + Backblaze B2

- Encrypted backups to B2
- Configurable retention (daily/weekly/monthly)
- SOPS-managed credentials
- Automatic pruning

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

configuration.nix

@@ -116,4 +116,15 @@
 
   # === CrowdSec ===
   myModules.crowdsec.enable = true;
+
+  # === Backups (Restic + B2) ===
+  # myModules.backup = {
+  #   enable = true;
+  #   repository = "b2:your-bucket-name";
+  #   paths = [ "/var/lib/bitwarden_rs" "/var/backup/vaultwarden" ];
+  # };
+  # Add to secrets.yaml:
+  # b2_account_id: "your-key-id"
+  # b2_account_key: "your-key"
+  # restic_password: "strong-encryption-password"
 }