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/models/task.py

@@ -47,3 +47,4 @@ class Task(Base):
     # Collaboration relationships
     comments = relationship("Comment", back_populates="task", cascade="all, delete-orphan")
     blockers = relationship("Blocker", back_populates="task", cascade="all, delete-orphan")
+    attachments = relationship("Attachment", back_populates="task", cascade="all, delete-orphan")