Store SHA256 checksums with artifacts and add multiple hash support

- Add sha256 field to API responses as explicit alias of artifact id
- Add checksum_sha1 and s3_etag fields to artifacts table
- Compute MD5, SHA1, and capture S3 ETag during upload
- Update StorageResult to return all checksums from storage layer
- Add migration 003_checksum_fields.sql for existing databases
- Add Dockerfile.local and docker-compose.local.yml for local development
- Update schemas to include all checksum fields in responses

backend/app/schemas.py

@@ -99,9 +99,13 @@ class PackageDetailResponse(BaseModel):
 # Artifact schemas
 class ArtifactResponse(BaseModel):
     id: str
+    sha256: str  # Explicit SHA256 field (same as id)
     size: int
     content_type: Optional[str]
     original_name: Optional[str]
+    checksum_md5: Optional[str] = None
+    checksum_sha1: Optional[str] = None
+    s3_etag: Optional[str] = None
     created_at: datetime
     created_by: str
     ref_count: int
@@ -173,9 +177,13 @@ class ArtifactTagInfo(BaseModel):
 class ArtifactDetailResponse(BaseModel):
     """Artifact with list of tags/packages referencing it"""
     id: str
+    sha256: str  # Explicit SHA256 field (same as id)
     size: int
     content_type: Optional[str]
     original_name: Optional[str]
+    checksum_md5: Optional[str] = None
+    checksum_sha1: Optional[str] = None
+    s3_etag: Optional[str] = None
     created_at: datetime
     created_by: str
     ref_count: int
@@ -189,9 +197,13 @@ class ArtifactDetailResponse(BaseModel):
 class PackageArtifactResponse(BaseModel):
     """Artifact with tags for package artifact listing"""
     id: str
+    sha256: str  # Explicit SHA256 field (same as id)
     size: int
     content_type: Optional[str]
     original_name: Optional[str]
+    checksum_md5: Optional[str] = None
+    checksum_sha1: Optional[str] = None
+    s3_etag: Optional[str] = None
     created_at: datetime
     created_by: str
     format_metadata: Optional[Dict[str, Any]] = None
@@ -204,10 +216,14 @@ class PackageArtifactResponse(BaseModel):
 # Upload response
 class UploadResponse(BaseModel):
     artifact_id: str
+    sha256: str  # Explicit SHA256 field (same as artifact_id)
     size: int
     project: str
     package: str
     tag: Optional[str]
+    checksum_md5: Optional[str] = None
+    checksum_sha1: Optional[str] = None
+    s3_etag: Optional[str] = None
     format_metadata: Optional[Dict[str, Any]] = None
     deduplicated: bool = False