feat: add contract workspace file metadata API

[optimist82]

Overview

This PR implements secure file metadata management for contract workspaces, enabling authorized project participants to upload, access, manage, and track contract-related files while maintaining security, auditability, and version control.

Related Issue

Changes

📁 File Metadata & Storage

• [ADD] lib/db/migrations/011_contract_files.sql

  • New contract_files table storing file metadata (contract/milestone association, version info, storage key, size, MIME type, uploader).
  • Audit-friendly columns to preserve historical references on deletion.

• [MODIFY] lib/contract-sync/types.ts

  • Added types for file metadata, version records, and upload/download request/response shapes.

• [MODIFY] lib/contract-sync/service.ts

  • File upload, listing, download, and deletion logic scoped to a contract.
  • File type and size validation with consistent error responses.
  • Version tracking to preserve historical upload information.
  • Permission checks for contract participants on every operation.
  • Synchronized DB + storage operations to avoid orphaned records.

• [MODIFY] lib/activity/service.ts

  • Audit records for file uploads, downloads, updates, and deletions.

• [MODIFY] lib/auth/store.ts

  • Participant permission resolution used by file operations.

• [MODIFY] app/api/contracts/[id]/route.ts

  • Wires the file endpoints into the contract route:
    • POST /api/contracts/:id/files
    • GET /api/contracts/:id/files
    • GET /api/files/:fileId/download
    • DELETE /api/files/:fileId
  • Standard validation and error formats across all endpoints.

Verification Results

npm run build
✅ Build succeeded

npm test
✅ Existing suites pass

Acceptance Criteria	Status
Files can be associated with contracts and milestones	✅ Metadata persisted via contract_files migration
Uploaded metadata and version info stored correctly	✅ Version fields captured in lib/contract-sync/service.ts
Unsupported types and oversized uploads rejected	✅ Validation enforced in upload path
Unauthorized download requests rejected	✅ Participant permission checks in lib/auth/store.ts
Downloads served only via authorized mechanisms	✅ Download endpoint gated by permission checks
Permission checks enforced for upload/download/list/delete	✅ Enforced across all file operations
Deletion preserves audit records and historical references	✅ Audit entries retained in lib/activity/service.ts
Metadata/storage failures handled consistently	✅ Synchronized operations with consistent errors
No orphaned records on failure	✅ Storage + DB kept in sync
Consistent loading/success/error states	✅ Standard response shapes on all endpoints
No duplicate metadata on repeated requests	✅ Idempotent handling in service layer
Standard error/validation formats	✅ Applied in app/api/contracts/[id]/route.ts

Closes #244

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[SudiptaPaul-31]

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