feat: implement custom fields, gantt view, calendar view, and file encryption

- Custom Fields (FEAT-001):
  - CustomField and TaskCustomValue models with formula support
  - CRUD API for custom field management
  - Formula engine for calculated fields
  - Frontend: CustomFieldEditor, CustomFieldInput, ProjectSettings page
  - Task list API now includes custom_values
  - KanbanBoard displays custom field values

- Gantt View (FEAT-003):
  - TaskDependency model with FS/SS/FF/SF dependency types
  - Dependency CRUD API with cycle detection
  - start_date field added to tasks
  - GanttChart component with Frappe Gantt integration
  - Dependency type selector in UI

- Calendar View (FEAT-004):
  - CalendarView component with FullCalendar integration
  - Date range filtering API for tasks
  - Drag-and-drop date updates
  - View mode switching in Tasks page

- File Encryption (FEAT-010):
  - AES-256-GCM encryption service
  - EncryptionKey model with key rotation support
  - Admin API for key management
  - Encrypted upload/download for confidential projects

- Migrations: 011 (custom fields), 012 (encryption keys), 013 (task dependencies)
- Updated issues.md with completion status

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

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

backend/.env.example

@@ -20,3 +20,14 @@ AUTH_API_URL=https://pj-auth-api.vercel.app
 
 # System Admin
 SYSTEM_ADMIN_EMAIL=ymirliu@panjit.com.tw
+
+# File Encryption (AES-256)
+# Master key for encrypting file encryption keys (optional - if not set, file encryption is disabled)
+# Generate a new key with:
+#   python -c "import secrets, base64; print(base64.urlsafe_b64encode(secrets.token_bytes(32)).decode())"
+#
+# IMPORTANT:
+# - Keep this key secure and back it up! If lost, encrypted files cannot be decrypted.
+# - Store backup in a secure location separate from the database backup.
+# - Do NOT change this key after files have been encrypted (use key rotation instead).
+ENCRYPTION_MASTER_KEY=