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/task_custom_value.py

@@ -0,0 +1,24 @@
+import uuid
+from sqlalchemy import Column, String, Text, DateTime, ForeignKey, UniqueConstraint
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class TaskCustomValue(Base):
+    __tablename__ = "pjctrl_task_custom_values"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    task_id = Column(String(36), ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"), nullable=False)
+    field_id = Column(String(36), ForeignKey("pjctrl_custom_fields.id", ondelete="CASCADE"), nullable=False)
+    value = Column(Text, nullable=True)  # Stored as text, parsed based on field_type
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    # Unique constraint: one value per task-field combination
+    __table_args__ = (
+        UniqueConstraint('task_id', 'field_id', name='uq_task_field'),
+    )
+
+    # Relationships
+    task = relationship("Task", back_populates="custom_values")
+    field = relationship("CustomField", back_populates="values")