Add upload/download tests for size boundaries and concurrency

- Add size boundary tests: 1B, 1KB, 10KB, 100KB, 1MB, 5MB, 10MB, 50MB
- Add large file tests (100MB-1GB) marked with @pytest.mark.large
- Add chunk boundary tests at 64KB boundaries
- Add concurrent upload/download tests (2, 5, 10 parallel)
- Add data integrity tests (binary, text, unicode, compressed)
- Add generate_content() and sized_content fixture for test helpers
- Add @pytest.mark.large and @pytest.mark.concurrent markers
- Fix Content-Disposition header encoding for non-ASCII filenames (RFC 5987)

backend/pytest.ini

@@ -12,6 +12,8 @@ markers =
     unit: Unit tests (no external dependencies)
     integration: Integration tests (require database/storage)
     slow: Slow tests (skip with -m "not slow")
+    large: Large file tests (100MB+, skip with -m "not large")
+    concurrent: Concurrent operation tests
 
 # Coverage configuration
 [coverage:run]