Upload workflow enhancements: S3 verification, timing, client checksum support (#19)

- Add S3 object verification after upload (size validation before DB commit)
- Add cleanup of S3 objects if DB commit fails
- Record upload duration_ms and user_agent
- Support X-Checksum-SHA256 header for client-side checksum verification
- Add already_existed flag to StorageResult for deduplication tracking
- Add status, error_message, client_checksum columns to Upload model
- Add UploadLock model for future 409 conflict detection
- Add consistency-check admin endpoint for detecting orphaned S3 objects
- Add migration 005_upload_enhancements.sql

backend/app/schemas.py

@@ -548,6 +548,19 @@ class StorageStatsResponse(BaseModel):
     storage_saved_bytes: int  # Bytes saved through deduplication
 
 
+class ConsistencyCheckResponse(BaseModel):
+    """Result of S3/Database consistency check"""
+
+    total_artifacts_checked: int
+    orphaned_s3_objects: int  # Objects in S3 but not in DB
+    missing_s3_objects: int  # Records in DB but not in S3
+    size_mismatches: int  # Records where DB size != S3 size
+    healthy: bool
+    orphaned_s3_keys: List[str] = []  # Limited list of orphaned S3 keys
+    missing_s3_keys: List[str] = []  # Limited list of missing S3 keys
+    size_mismatch_artifacts: List[Dict[str, Any]] = []  # Limited list of mismatches
+
+
 class DeduplicationStatsResponse(BaseModel):
     """Deduplication effectiveness statistics"""