Add download verification with SHA256 checksum support (#26, #27, #28, #29)

CHANGELOG.md

@@ -7,6 +7,30 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 ### Added
+- Added download verification with `verify` and `verify_mode` query parameters (#26)
+  - `?verify=true&verify_mode=pre` - Pre-verification: verify before streaming (guaranteed no corrupt data)
+  - `?verify=true&verify_mode=stream` - Streaming verification: verify while streaming (logs error if mismatch)
+- Added checksum response headers to all download endpoints (#27)
+  - `X-Checksum-SHA256` - SHA256 hash of the artifact
+  - `X-Content-Length` - File size in bytes
+  - `X-Checksum-MD5` - MD5 hash (if available)
+  - `ETag` - Artifact ID (SHA256)
+  - `Digest` - RFC 3230 format sha-256 hash (base64)
+  - `X-Verified` - Verification status (true/false/pending)
+- Added `checksum.py` module with SHA256 utilities (#26)
+  - `compute_sha256()` and `compute_sha256_stream()` functions
+  - `HashingStreamWrapper` for incremental hash computation
+  - `VerifyingStreamWrapper` for stream verification
+  - `verify_checksum()` and `verify_checksum_strict()` functions
+  - `ChecksumMismatchError` exception with context
+- Added `get_verified()` and `get_stream_verified()` methods to storage layer (#26)
+- Added `logging_config.py` module with structured logging (#28)
+  - JSON logging format for production
+  - Request ID tracking via context variables
+  - Verification failure logging with full context
+- Added `log_level` and `log_format` settings to configuration (#28)
+- Added 62 unit tests for checksum utilities and verification (#29)
+- Added 17 integration tests for download verification API (#29)
 - Added global artifacts endpoint `GET /api/v1/artifacts` with project/package/tag/size/date filters (#18)
 - Added global tags endpoint `GET /api/v1/tags` with project/package/search/date filters (#18)
 - Added wildcard pattern matching (`*`) for tag filters across all endpoints (#18)