fix: add build_runner step to Dockerfile and update CACHEBUST to v4

Add `dart run build_runner build --delete-conflicting-outputs` step before Flutter build to ensure all generated files (Riverpod and Hive) are fresh and properly generated. This ensures no stale or incompatible generated code causes build failures.

Also update CACHEBUST to v4 to force Docker to bypass cache and apply all recent fixes.

Dockerfile

@@ -19,7 +19,7 @@ RUN flutter config --enable-web
 # Copy dependency files first for better caching
 # Note: pubspec.lock will be generated by flutter pub get if missing
 # Cache buster - INCREMENT THIS VALUE TO FORCE REBUILD ON DEPLOY
-ARG CACHEBUST=2025-12-08-v3
+ARG CACHEBUST=2025-12-08-v4
 COPY pubspec.yaml ./
 
 # Get dependencies (generates pubspec.lock automatically)
@@ -31,6 +31,9 @@ COPY . .
 # Re-run pub get after copying source to ensure lockfile consistency
 RUN flutter pub get
 
+# Generate code (for Riverpod and Hive)
+RUN dart run build_runner build --delete-conflicting-outputs
+
 # Clean build environment to avoid conflicts
 RUN flutter clean