ci: persist Turbo cache across CI runs

Add an actions/cache@v4 step after pnpm install to save and restore
.turbo/cache between runs. Cache key is sha-scoped so each run saves a
fresh entry; restore-keys falls back to the most recent same-OS entry.
Benefits check-types (~110 packages); lint and test don't go through
Turbo. Follow-ups: extend to docker.yml; adopt Vercel Remote Cache or
route root lint/test through turbo for broader wins.

Author: Medformatik

.github/workflows/ci.yml

@@ -35,6 +35,16 @@ jobs:
 
       - run: pnpm install --frozen-lockfile
 
+      # Cache benefits check-types only (lint/test don't run through turbo);
+      # revisit if root scripts move under turbo.
+      - name: Restore Turbo cache
+        uses: actions/cache@v4
+        with:
+          path: .turbo/cache
+          key: turbo-${{ runner.os }}-${{ github.sha }}
+          restore-keys: |
+            turbo-${{ runner.os }}-
+
       - name: Lint commit messages (PR only)
         if: github.event_name == 'pull_request'
         run: pnpm exec commitlint --from "${BASE_SHA}" --to "${HEAD_SHA}" --verbose