perf: cache Android SDK, Gradle, and pip in CI

Added actions/cache@v3 for:
- ~/android-sdk (keyed on OS + API 34) — skips 150MB download
- ~/.gradle + android/.gradle-home (keyed on OS + gradle 8.7
  + gradle files hash) — skips 130MB gradle download + all
  dependency resolution and jar caching
- ~/.cache/pip (keyed on requirements.txt hash) — skips
  flask/click/werkzeug installs

Build step now checks if cached paths exist before downloading.
Expected speedup: 5-6 min → ~1-2 min on cache hits.

.gitea/workflows/ci.yml

@@ -16,6 +16,12 @@ jobs:
         with:
           python-version: "3.13"
 
+      - name: Cache pip
+        uses: actions/cache@v3
+        with:
+          path: ~/.cache/pip
+          key: pip-${{ runner.os }}-${{ hashFiles('requirements.txt') }}
+
       - name: Install dependencies
         run: pip install -r requirements.txt
 
@@ -52,17 +58,39 @@ jobs:
           distribution: temurin
           java-version: "17"
 
+      - name: Cache Android SDK
+        uses: actions/cache@v3
+        with:
+          path: ~/android-sdk
+          key: android-sdk-${{ runner.os }}-34
+
+      - name: Cache Gradle
+        uses: actions/cache@v3
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+            android/.gradle-home
+          key: gradle-${{ runner.os }}-8.7-${{ hashFiles('android/**/*.gradle*') }}
+          restore-keys: |
+            gradle-${{ runner.os }}-8.7-
+            gradle-${{ runner.os }}-
+
       - name: Build APK
         run: |
           export ANDROID_SDK_ROOT="$HOME/android-sdk"
-          mkdir -p "$ANDROID_SDK_ROOT"
 
+          # Download SDK command-line tools only if not cached
+          if [ ! -f "$ANDROID_SDK_ROOT/cmdline-tools/latest/bin/sdkmanager" ]; then
+            mkdir -p "$ANDROID_SDK_ROOT"
             wget -q https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip \
               -O /tmp/cmdline-tools.zip
             unzip -qo /tmp/cmdline-tools.zip -d /tmp/cmdline-tools-tmp
             mkdir -p "$ANDROID_SDK_ROOT/cmdline-tools"
             mv /tmp/cmdline-tools-tmp/cmdline-tools "$ANDROID_SDK_ROOT/cmdline-tools/latest"
+          fi
 
+          # Accept licenses + install packages (idempotent — skips if already installed)
           { yes 2>/dev/null || true; } | "$ANDROID_SDK_ROOT/cmdline-tools/latest/bin/sdkmanager" --licenses > /dev/null 2>&1 || true
           "$ANDROID_SDK_ROOT/cmdline-tools/latest/bin/sdkmanager" \
             "platform-tools" \
@@ -71,14 +99,21 @@ jobs:
 
           echo "sdk.dir=$ANDROID_SDK_ROOT" > android/local.properties
 
+          # Download Gradle only if not cached
+          if [ ! -d "android/.gradle-home/gradle-8.7" ]; then
             wget -q https://services.gradle.org/distributions/gradle-8.7-bin.zip -O /tmp/gradle.zip
             mkdir -p android/.gradle-home
             unzip -qo /tmp/gradle.zip -d android/.gradle-home
+          fi
 
+          # Generate wrapper only if needed
+          if [ ! -f android/gradlew ]; then
             cd android
             .gradle-home/gradle-8.7/bin/gradle wrapper --gradle-version 8.7 --no-daemon
-          ./gradlew assembleDebug --no-daemon
             cd ..
+          fi
+
+          cd android && ./gradlew assembleDebug --no-daemon && cd ..
           cp android/app/build/outputs/apk/debug/app-debug.apk meal-tracker.apk
 
       - name: Upload APK artifact