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/custom_field.py

@@ -0,0 +1,88 @@
+from pydantic import BaseModel, Field, field_validator
+from typing import Optional, List, Any, Dict
+from datetime import datetime
+from enum import Enum
+
+
+class FieldType(str, Enum):
+    TEXT = "text"
+    NUMBER = "number"
+    DROPDOWN = "dropdown"
+    DATE = "date"
+    PERSON = "person"
+    FORMULA = "formula"
+
+
+class CustomFieldBase(BaseModel):
+    name: str = Field(..., min_length=1, max_length=100)
+    field_type: FieldType
+    options: Optional[List[str]] = None  # For dropdown type
+    formula: Optional[str] = None  # For formula type
+    is_required: bool = False
+
+    @field_validator('options')
+    @classmethod
+    def validate_options(cls, v, info):
+        field_type = info.data.get('field_type')
+        if field_type == FieldType.DROPDOWN:
+            if not v or len(v) == 0:
+                raise ValueError('Dropdown fields must have at least one option')
+            if len(v) > 50:
+                raise ValueError('Dropdown fields can have at most 50 options')
+        return v
+
+    @field_validator('formula')
+    @classmethod
+    def validate_formula(cls, v, info):
+        field_type = info.data.get('field_type')
+        if field_type == FieldType.FORMULA and not v:
+            raise ValueError('Formula fields must have a formula expression')
+        return v
+
+
+class CustomFieldCreate(CustomFieldBase):
+    pass
+
+
+class CustomFieldUpdate(BaseModel):
+    name: Optional[str] = Field(None, min_length=1, max_length=100)
+    options: Optional[List[str]] = None
+    formula: Optional[str] = None
+    is_required: Optional[bool] = None
+
+
+class CustomFieldResponse(CustomFieldBase):
+    id: str
+    project_id: str
+    position: int
+    created_at: datetime
+    updated_at: datetime
+
+    class Config:
+        from_attributes = True
+
+
+class CustomFieldListResponse(BaseModel):
+    fields: List[CustomFieldResponse]
+    total: int
+
+
+# Task custom value schemas
+class CustomValueInput(BaseModel):
+    field_id: str
+    value: Optional[Any] = None  # Can be string, number, date string, or user id
+
+
+class CustomValueResponse(BaseModel):
+    field_id: str
+    field_name: str
+    field_type: FieldType
+    value: Optional[Any] = None
+    display_value: Optional[str] = None  # Formatted for display
+
+    class Config:
+        from_attributes = True
+
+
+class TaskCustomValuesUpdate(BaseModel):
+    custom_values: List[CustomValueInput]