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/models/custom_field.py

@@ -0,0 +1,37 @@
+import uuid
+import enum
+from sqlalchemy import Column, String, Text, Boolean, DateTime, ForeignKey, Enum, JSON, Integer
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class FieldType(str, enum.Enum):
+    TEXT = "text"
+    NUMBER = "number"
+    DROPDOWN = "dropdown"
+    DATE = "date"
+    PERSON = "person"
+    FORMULA = "formula"
+
+
+class CustomField(Base):
+    __tablename__ = "pjctrl_custom_fields"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    project_id = Column(String(36), ForeignKey("pjctrl_projects.id", ondelete="CASCADE"), nullable=False)
+    name = Column(String(100), nullable=False)
+    field_type = Column(
+        Enum("text", "number", "dropdown", "date", "person", "formula", name="field_type_enum"),
+        nullable=False
+    )
+    options = Column(JSON, nullable=True)  # For dropdown: list of options
+    formula = Column(Text, nullable=True)  # For formula: formula expression
+    is_required = Column(Boolean, default=False, nullable=False)
+    position = Column(Integer, default=0, nullable=False)  # For ordering fields
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    # Relationships
+    project = relationship("Project", back_populates="custom_fields")
+    values = relationship("TaskCustomValue", back_populates="field", cascade="all, delete-orphan")