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/tests/conftest.py

@@ -32,6 +32,8 @@ from tests.factories import (
     compute_md5,
     compute_sha1,
     upload_test_file,
+    generate_content,
+    generate_content_with_hash,
     TEST_CONTENT_HELLO,
     TEST_HASH_HELLO,
     TEST_MD5_HELLO,
@@ -271,3 +273,41 @@ def test_content():
     content = f"test-content-{uuid.uuid4().hex}".encode()
     sha256 = compute_sha256(content)
     return (content, sha256)
+
+
+@pytest.fixture
+def sized_content():
+    """
+    Factory fixture for generating content of specific sizes.
+
+    Usage:
+        def test_example(sized_content):
+            content, hash = sized_content(1024)  # 1KB
+            content, hash = sized_content(1024 * 1024)  # 1MB
+    """
+    def _generate(size: int, seed: int = None):
+        return generate_content_with_hash(size, seed)
+    return _generate
+
+
+# =============================================================================
+# Size Constants for Tests
+# =============================================================================
+
+# Common file sizes for boundary testing
+SIZE_1B = 1
+SIZE_1KB = 1024
+SIZE_10KB = 10 * 1024
+SIZE_100KB = 100 * 1024
+SIZE_1MB = 1024 * 1024
+SIZE_5MB = 5 * 1024 * 1024
+SIZE_10MB = 10 * 1024 * 1024
+SIZE_50MB = 50 * 1024 * 1024
+SIZE_100MB = 100 * 1024 * 1024
+SIZE_250MB = 250 * 1024 * 1024
+SIZE_500MB = 500 * 1024 * 1024
+SIZE_1GB = 1024 * 1024 * 1024
+
+# Chunk size boundaries (based on typical S3 multipart chunk sizes)
+CHUNK_SIZE = 64 * 1024  # 64KB typical chunk
+MULTIPART_THRESHOLD = 100 * 1024 * 1024  # 100MB multipart threshold