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/factories.py

@@ -97,6 +97,7 @@ def upload_test_file(
     content: bytes,
     filename: str = "test.bin",
     tag: Optional[str] = None,
+    version: Optional[str] = None,
 ) -> dict:
     """
     Helper function to upload a test file via the API.
@@ -108,6 +109,7 @@ def upload_test_file(
         content: File content as bytes
         filename: Original filename
         tag: Optional tag to assign
+        version: Optional version to assign
 
     Returns:
         The upload response as a dict
@@ -116,6 +118,8 @@ def upload_test_file(
     data = {}
     if tag:
         data["tag"] = tag
+    if version:
+        data["version"] = version
 
     response = client.post(
         f"/api/v1/project/{project}/{package}/upload",
@@ -126,6 +130,41 @@ def upload_test_file(
     return response.json()
 
 
+def generate_content(size: int, seed: Optional[int] = None) -> bytes:
+    """
+    Generate deterministic or random content of a specified size.
+
+    Args:
+        size: Size of content in bytes
+        seed: Optional seed for reproducible content (None for random)
+
+    Returns:
+        Bytes of the specified size
+    """
+    if size == 0:
+        return b""
+    if seed is not None:
+        import random
+        rng = random.Random(seed)
+        return bytes(rng.randint(0, 255) for _ in range(size))
+    return os.urandom(size)
+
+
+def generate_content_with_hash(size: int, seed: Optional[int] = None) -> tuple[bytes, str]:
+    """
+    Generate content of specified size and compute its SHA256 hash.
+
+    Args:
+        size: Size of content in bytes
+        seed: Optional seed for reproducible content
+
+    Returns:
+        Tuple of (content_bytes, sha256_hash)
+    """
+    content = generate_content(size, seed)
+    return content, compute_sha256(content)
+
+
 # =============================================================================
 # Project/Package Factories
 # =============================================================================