Rename service to EngPadSyncService (buf lint), add java_package, add buf.yaml

- Proto service renamed from EngPadSync to EngPadSyncService per buf
  STANDARD lint rule SERVICE_SUFFIX
- Added java_package and java_multiple_files options for Android client
- Added buf.yaml with STANDARD lint and FILE breaking detection
- Regenerated Go gRPC stubs, updated server references

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/grpcserver/sync.go

@@ -12,7 +12,7 @@ import (
 )
 
 type SyncService struct {
-	pb.UnimplementedEngPadSyncServer
+	pb.UnimplementedEngPadSyncServiceServer
 	DB      *sql.DB
 	BaseURL string
 }