refactor: split server modules into smaller more manageable files

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

@@ -0,0 +1,41 @@
+{
+  lib,
+  config,
+  ...
+}: {
+  config = lib.mkIf config.services.forgejo.enable (
+    lib.mkMerge [
+      {
+        host = {
+          postgres = {
+            enable = true;
+          };
+        };
+
+        assertions = [
+          {
+            assertion = config.services.forgejo.settings.database.DB_TYPE == "postgres";
+            message = "Forgejo database type must be postgres";
+          }
+        ];
+      }
+      (lib.mkIf config.host.postgres.enable {
+        host = {
+          postgres = {
+            extraUsers = {
+              forgejo = {
+                isClient = true;
+                createUser = true;
+              };
+            };
+            extraDatabases = {
+              forgejo = {
+                name = "forgejo";
+              };
+            };
+          };
+        };
+      })
+    ]
+  );
+}