ci: cache Maven repository explicitly for both Java jobs

The built-in cache: maven in setup-java@v4 does not reliably work
on self-hosted act runners. Replace with an explicit actions/cache@v4
on ~/.m2/repository keyed on pom.xml hash.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.gitea/workflows/ci.yml

@@ -69,7 +69,13 @@ jobs:
         with:
           java-version: '21'
           distribution: temurin
-          cache: maven
+
+      - name: Cache Maven repository
+        uses: actions/cache@v4
+        with:
+          path: ~/.m2/repository
+          key: maven-${{ hashFiles('backend/pom.xml') }}
+          restore-keys: maven-
 
       - name: Run backend tests
         run: |
@@ -118,7 +124,13 @@ jobs:
         with:
           java-version: '21'
           distribution: temurin
-          cache: maven
+
+      - name: Cache Maven repository
+        uses: actions/cache@v4
+        with:
+          path: ~/.m2/repository
+          key: maven-${{ hashFiles('backend/pom.xml') }}
+          restore-keys: maven-
 
       - name: Build backend (skip tests — covered by separate Java test job)
         run: |