scalafmt: use sbt plugin, add to tests workflow

.github/workflows/test.yml

@@ -22,3 +22,13 @@ jobs:
     - name: Run tests
       run: |
         nix-shell --command "sbt -DSnippetsContainerTest.mongo=true cachedCiTest"
+  formatting:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@v4
+    - uses: cachix/install-nix-action@v24
+      with:
+        nix_path: nixpkgs=channel:nixos-22.11
+    - name: Check formatting
+      run: |
+        nix-shell --command "sbt scalafmtCheckAll"

CONTRIBUTING.md

@@ -60,7 +60,7 @@ open `http://localhost:9000`
 
 ## Scalafmt
 
-Make sure to run `bin/scalafmt` to format your code.
+Make sure to run `sbt scalafmtAll` to format your code.
 
 You can install a pre-commit hook with `bin/hooks.sh`
 

project/plugins.sbt

@@ -1,3 +1,5 @@
+addSbtPlugin("org.scalameta" % "sbt-scalafmt" % "2.5.4")
+
 addSbtPlugin("com.eed3si9n"      % "sbt-assembly"        % "2.1.5")
 addSbtPlugin("com.github.sbt"    % "sbt-native-packager" % "1.9.16")
 addSbtPlugin("io.spray"          % "sbt-revolver"        % "0.10.0")