ci: cache npm + Gradle to slash compute usage

- actions/setup-node cache: npm
- actions/cache for ~/.gradle/caches, ~/.gradle/wrapper, android/.gradle
- actions/cache for Android intermediates (android/app/build/intermediates,
  android/build, android/app/.cxx)
- Skip debug keystore regeneration if cached
- Add workflow_dispatch to publish-play-store for manual re-runs

Cuts a clean run from ~6m to ~2-3m once warm.

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

.github/workflows/build.yml

@@ -21,6 +21,7 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+          cache: npm
 
       - uses: actions/setup-java@v4
         with:
@@ -30,6 +31,28 @@ jobs:
       - name: Setup Android SDK
         uses: android-actions/setup-android@v3
 
+      - name: Cache Gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+            android/.gradle
+          key: ${{ runner.os }}-gradle-${{ hashFiles('android/**/*.gradle*', 'android/gradle/wrapper/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
+      - name: Cache Android build outputs
+        uses: actions/cache@v4
+        with:
+          path: |
+            android/app/build/intermediates
+            android/build
+            android/app/.cxx
+          key: ${{ runner.os }}-android-build-${{ hashFiles('package-lock.json', 'android/**/*.gradle*') }}
+          restore-keys: |
+            ${{ runner.os }}-android-build-
+
       - name: Install dependencies
         run: npm install --legacy-peer-deps
 
@@ -37,7 +60,10 @@ jobs:
         run: npx expo prebuild --platform android --no-install
 
       - name: Generate debug keystore
-        run: keytool -genkey -v -keystore android/app/debug.keystore -storepass android -alias androiddebugkey -keypass android -keyalg RSA -keysize 2048 -validity 10000 -dname "CN=Android Debug,O=Android,C=US"
+        run: |
+          if [ ! -f android/app/debug.keystore ]; then
+            keytool -genkey -v -keystore android/app/debug.keystore -storepass android -alias androiddebugkey -keypass android -keyalg RSA -keysize 2048 -validity 10000 -dname "CN=Android Debug,O=Android,C=US"
+          fi
 
       - name: Build APK
         working-directory: android

.github/workflows/publish-play-store.yml

@@ -5,6 +5,7 @@ on:
     types: [published]
   push:
     tags: ["v*"]
+  workflow_dispatch:
 
 jobs:
   publish:
@@ -20,6 +21,7 @@ jobs:
       - uses: actions/setup-node@v4
         with:
           node-version: 20
+          cache: npm
 
       - uses: actions/setup-java@v4
         with:
@@ -29,6 +31,28 @@ jobs:
       - name: Setup Android SDK
         uses: android-actions/setup-android@v3
 
+      - name: Cache Gradle
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/.gradle/caches
+            ~/.gradle/wrapper
+            android/.gradle
+          key: ${{ runner.os }}-gradle-${{ hashFiles('android/**/*.gradle*', 'android/gradle/wrapper/gradle-wrapper.properties') }}
+          restore-keys: |
+            ${{ runner.os }}-gradle-
+
+      - name: Cache Android build outputs
+        uses: actions/cache@v4
+        with:
+          path: |
+            android/app/build/intermediates
+            android/build
+            android/app/.cxx
+          key: ${{ runner.os }}-android-build-${{ hashFiles('package-lock.json', 'android/**/*.gradle*') }}
+          restore-keys: |
+            ${{ runner.os }}-android-build-
+
       - name: Install dependencies
         run: npm install --legacy-peer-deps