feat: implement document management module

- Backend (FastAPI):
  - Attachment and AttachmentVersion models with migration
  - FileStorageService with SHA-256 checksum validation
  - File type validation (whitelist/blacklist)
  - Full CRUD API with version control support
  - Audit trail integration for upload/download/delete
  - Configurable upload directory and file size limit

- Frontend (React + Vite):
  - AttachmentUpload component with drag & drop
  - AttachmentList component with download/delete
  - TaskAttachments combined component
  - Attachments service for API calls

- Testing:
  - 12 tests for storage service and API endpoints

- OpenSpec:
  - add-document-management change archived

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/schemas/attachment.py

@@ -0,0 +1,50 @@
+from pydantic import BaseModel
+from typing import Optional, List
+from datetime import datetime
+
+
+class AttachmentVersionResponse(BaseModel):
+    id: str
+    version: int
+    file_size: int
+    checksum: str
+    uploaded_by: Optional[str] = None
+    uploader_name: Optional[str] = None
+    created_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class AttachmentResponse(BaseModel):
+    id: str
+    task_id: str
+    filename: str
+    original_filename: str
+    mime_type: str
+    file_size: int
+    current_version: int
+    is_encrypted: bool
+    uploaded_by: Optional[str] = None
+    uploader_name: Optional[str] = None
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class AttachmentListResponse(BaseModel):
+    attachments: List[AttachmentResponse]
+    total: int
+
+
+class AttachmentDetailResponse(AttachmentResponse):
+    versions: List[AttachmentVersionResponse] = []
+
+
+class VersionHistoryResponse(BaseModel):
+    attachment_id: str
+    filename: str
+    versions: List[AttachmentVersionResponse]
+    total: int