refactor: Move network configurations to machine directories

- Move network-congenital-optimist.nix to machines/congenital-optimist/
- Move network-sleeper-service.nix to machines/sleeper-service/
- Update import paths in machine configurations
- Clean up modules/network/common.nix to remove SSH duplication
- Consolidate SSH configuration in modules/security/ssh-keys.nix
- Remove machine-specific networking from shared common module

This improves dependency tracking by co-locating machine-specific
network configurations with their respective machines.

machines/sleeper-service/configuration.nix

@@ -4,9 +4,9 @@
     # Security modules
     ../../modules/security/ssh-keys.nix
     # Network configuration
-    ../../modules/network/network-sleeper-service.nix
+    ./network-sleeper-service.nix
     # Services
-    ../../modules/services/nfs.nix
+    ./nfs.nix
     ../../modules/system/transmission.nix
 
     # User modules - server only needs sma user