refactor: split server modules into smaller more manageable files

modules/nixos-modules/server/immich/database.nix

@@ -0,0 +1,26 @@
+{
+  lib,
+  config,
+  ...
+}: {
+  config = lib.mkIf config.services.immich.enable (lib.mkMerge [
+    {
+      host = {
+        postgres = {
+          enable = true;
+        };
+      };
+    }
+    (lib.mkIf config.host.postgres.enable {
+      host = {
+        postgres = {
+          extraUsers = {
+            ${config.services.immich.database.user} = {
+              isClient = true;
+            };
+          };
+        };
+      };
+    })
+  ]);
+}