Add dependency caching to PR check workflows

- Add bun dependency cache to pr-check.yml using actions/cache@v4
- Enable uv built-in cache in pr-check-backend.yml

This reduces CI run times by caching downloaded dependencies between runs.

Author: ukanga

.github/workflows/pr-check-backend.yml

@@ -26,6 +26,9 @@ jobs:
 
       - name: Install uv
         uses: astral-sh/setup-uv@v4
+        with:
+          enable-cache: true
+          cache-dependency-glob: "truecover-backend/uv.lock"
 
       - name: Set up Python
         run: uv python install 3.12

.github/workflows/pr-check.yml

@@ -29,6 +29,14 @@ jobs:
         with:
           bun-version: latest
 
+      - name: Cache bun dependencies
+        uses: actions/cache@v4
+        with:
+          path: ~/.bun/install/cache
+          key: ${{ runner.os }}-bun-${{ hashFiles('truecover-app/bun.lockb') }}
+          restore-keys: |
+            ${{ runner.os }}-bun-
+
       - name: Install dependencies
         run: bun install --frozen-lockfile