ci: add Go and npm dependency caching via actions/cache

Add actions/cache@v4 to all four CI jobs. Go module cache (go/pkg/mod + go-build) keyed on go.sum for test, race, and vulns jobs. npm cache (~/.npm + node_modules) keyed on package-lock.json for frontend job. Cuts dependency download time to near-zero on cache hit.

Ultraworked with [Sisyphus](https://github.com/code-yeongyu/oh-my-openagent)

Co-authored-by: Sisyphus <clio-agent@sisyphuslabs.ai>

.gitea/workflows/ci.yaml

@@ -40,6 +40,16 @@ jobs:
           go-version: "1.25"
           cache: false
 
+      - name: Cache Go modules
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/go/pkg/mod
+            ~/.cache/go-build
+          key: ${{ runner.os }}-go-${{ hashFiles('backend/go.sum') }}-test
+          restore-keys: |
+            ${{ runner.os }}-go-
+
       - name: Fix node toolcache path for Post-step cleanup
         run: |
           mkdir -p /opt/hostedtoolcache/node/22.23.1/x64/bin
@@ -95,6 +105,16 @@ jobs:
           go-version: "1.25"
           cache: false
 
+      - name: Cache Go modules
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/go/pkg/mod
+            ~/.cache/go-build
+          key: ${{ runner.os }}-go-${{ hashFiles('backend/go.sum') }}-race
+          restore-keys: |
+            ${{ runner.os }}-go-
+
       - name: Fix node toolcache path for Post-step cleanup
         run: |
           mkdir -p /opt/hostedtoolcache/node/22.23.1/x64/bin
@@ -132,6 +152,16 @@ jobs:
           go-version: "1.25"
           cache: false
 
+      - name: Cache Go modules
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/go/pkg/mod
+            ~/.cache/go-build
+          key: ${{ runner.os }}-go-${{ hashFiles('backend/go.sum') }}-vulns
+          restore-keys: |
+            ${{ runner.os }}-go-
+
       - name: Fix node toolcache path for Post-step cleanup
         run: |
           mkdir -p /opt/hostedtoolcache/node/22.23.1/x64/bin
@@ -157,6 +187,16 @@ jobs:
           mkdir -p /opt/hostedtoolcache/node/22.23.1/x64/bin
           ln -sf /usr/local/bin/node /opt/hostedtoolcache/node/22.23.1/x64/bin/node
 
+      - name: Cache npm dependencies
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.npm
+            frontend/node_modules
+          key: ${{ runner.os }}-npm-${{ hashFiles('frontend/package-lock.json') }}
+          restore-keys: |
+            ${{ runner.os }}-npm-
+
       - name: Frontend type check
         run: |
           cd frontend