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/schemas/task_dependency.py

@@ -0,0 +1,78 @@
+from pydantic import BaseModel, field_validator
+from typing import Optional, List
+from datetime import datetime
+from enum import Enum
+
+
+class DependencyType(str, Enum):
+    """Task dependency types for Gantt chart."""
+    FS = "FS"  # Finish-to-Start (most common)
+    SS = "SS"  # Start-to-Start
+    FF = "FF"  # Finish-to-Finish
+    SF = "SF"  # Start-to-Finish (rare)
+
+
+class TaskDependencyCreate(BaseModel):
+    """Schema for creating a task dependency."""
+    predecessor_id: str
+    dependency_type: DependencyType = DependencyType.FS
+    lag_days: int = 0
+
+    @field_validator('lag_days')
+    @classmethod
+    def validate_lag_days(cls, v):
+        if v < -365 or v > 365:
+            raise ValueError('lag_days must be between -365 and 365')
+        return v
+
+
+class TaskDependencyUpdate(BaseModel):
+    """Schema for updating a task dependency."""
+    dependency_type: Optional[DependencyType] = None
+    lag_days: Optional[int] = None
+
+    @field_validator('lag_days')
+    @classmethod
+    def validate_lag_days(cls, v):
+        if v is not None and (v < -365 or v > 365):
+            raise ValueError('lag_days must be between -365 and 365')
+        return v
+
+
+class TaskInfo(BaseModel):
+    """Brief task information for dependency response."""
+    id: str
+    title: str
+    start_date: Optional[datetime] = None
+    due_date: Optional[datetime] = None
+
+    class Config:
+        from_attributes = True
+
+
+class TaskDependencyResponse(BaseModel):
+    """Schema for task dependency response."""
+    id: str
+    predecessor_id: str
+    successor_id: str
+    dependency_type: DependencyType
+    lag_days: int
+    created_at: datetime
+    predecessor: Optional[TaskInfo] = None
+    successor: Optional[TaskInfo] = None
+
+    class Config:
+        from_attributes = True
+
+
+class TaskDependencyListResponse(BaseModel):
+    """Schema for list of task dependencies."""
+    dependencies: List[TaskDependencyResponse]
+    total: int
+
+
+class DependencyValidationError(BaseModel):
+    """Schema for dependency validation error details."""
+    error_type: str  # 'circular', 'self_reference', 'duplicate', 'cross_project'
+    message: str
+    details: Optional[dict] = None