CI: slim Flutter SDK for the interop job; APK only on app changes, Gradle cached

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.github/workflows/python-tests.yml

@@ -69,11 +69,10 @@ jobs:
           uv pip install --quiet --python .freedays-venv/bin/python .utils/crdt-sync
           uv pip install --quiet --python .freedays-venv/bin/python --no-deps .utils/freedays
 
-      - uses: subosito/flutter-action@v2
+      - uses: kuhyx/utils/flutter-setup@flutter-setup-v1
         with:
           flutter-version: 3.47.6
-          channel: stable
-          cache: true
+          working-directory: phone_app
 
       - name: Dart writes, Python replays, both must agree
         run: FREEDAYS_PYTHON="$PWD/.freedays-venv/bin/python" bash scripts/free_days_interop/check.sh

.github/workflows/release-apk.yml

@@ -3,6 +3,14 @@ name: release-apk
 on:
   push:
     branches: [main]
+    # A publisher, not a gate: build only when something that ends up in
+    # the APK changed. versionCode is the commit count, so skipped pushes
+    # still leave it monotonic.
+    paths:
+      - 'phone_app/**'
+      - '.fvmrc'
+      - '.versioncode-offset'
+      - '.github/workflows/release-apk.yml'
   # Lets a release be rebuilt from the current main without a dummy commit.
   workflow_dispatch:
 
@@ -74,6 +82,8 @@ jobs:
         with:
           distribution: temurin
           java-version: "17"
+          # ~/.gradle caches + wrapper, keyed on the Gradle files.
+          cache: gradle
 
       - uses: subosito/flutter-action@v2
         with: