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.py

@@ -30,6 +30,7 @@ class Task(Base):
     original_estimate = Column(Numeric(8, 2), nullable=True)
     time_spent = Column(Numeric(8, 2), default=0, nullable=False)
     blocker_flag = Column(Boolean, default=False, nullable=False)
+    start_date = Column(DateTime, nullable=True)
     due_date = Column(DateTime, nullable=True)
     position = Column(Integer, default=0, nullable=False)
     created_by = Column(String(36), ForeignKey("pjctrl_users.id"), nullable=False)
@@ -55,3 +56,18 @@ class Task(Base):
     blockers = relationship("Blocker", back_populates="task", cascade="all, delete-orphan")
     attachments = relationship("Attachment", back_populates="task", cascade="all, delete-orphan")
     trigger_logs = relationship("TriggerLog", back_populates="task")
+    custom_values = relationship("TaskCustomValue", back_populates="task", cascade="all, delete-orphan")
+
+    # Dependency relationships (for Gantt view)
+    predecessors = relationship(
+        "TaskDependency",
+        foreign_keys="TaskDependency.successor_id",
+        back_populates="successor",
+        cascade="all, delete-orphan"
+    )
+    successors = relationship(
+        "TaskDependency",
+        foreign_keys="TaskDependency.predecessor_id",
+        back_populates="predecessor",
+        cascade="all, delete-orphan"
+    )