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/app/core/config.py

@@ -1,6 +1,6 @@
 from pydantic_settings import BaseSettings
 from pydantic import field_validator
-from typing import List
+from typing import List, Optional
 import os
 
 
@@ -52,6 +52,35 @@ class Settings(BaseSettings):
             )
         return v
 
+    # Encryption - Master key for encrypting file encryption keys
+    # Must be a 32-byte (256-bit) key encoded as base64 for AES-256
+    # Generate with: python -c "import secrets, base64; print(base64.urlsafe_b64encode(secrets.token_bytes(32)).decode())"
+    ENCRYPTION_MASTER_KEY: Optional[str] = None
+
+    @field_validator("ENCRYPTION_MASTER_KEY")
+    @classmethod
+    def validate_encryption_master_key(cls, v: Optional[str]) -> Optional[str]:
+        """Validate that ENCRYPTION_MASTER_KEY is properly formatted if set."""
+        if v is None or v.strip() == "":
+            return None
+        # Basic validation - should be base64 encoded 32 bytes
+        import base64
+        try:
+            decoded = base64.urlsafe_b64decode(v)
+            if len(decoded) != 32:
+                raise ValueError(
+                    "ENCRYPTION_MASTER_KEY must be a base64-encoded 32-byte key. "
+                    "Generate with: python -c \"import secrets, base64; print(base64.urlsafe_b64encode(secrets.token_bytes(32)).decode())\""
+                )
+        except Exception as e:
+            if "must be a base64-encoded" in str(e):
+                raise
+            raise ValueError(
+                "ENCRYPTION_MASTER_KEY must be a valid base64-encoded string. "
+                f"Error: {e}"
+            )
+        return v
+
     # External Auth API
     AUTH_API_URL: str = "https://pj-auth-api.vercel.app"