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_dependency.py

@@ -0,0 +1,68 @@
+from sqlalchemy import Column, String, Integer, Enum, DateTime, ForeignKey, UniqueConstraint
+from sqlalchemy.orm import relationship
+from sqlalchemy.sql import func
+from app.core.database import Base
+import enum
+
+
+class DependencyType(str, enum.Enum):
+    """
+    Task dependency types for Gantt chart.
+
+    FS (Finish-to-Start): Predecessor must finish before successor starts (most common)
+    SS (Start-to-Start): Predecessor must start before successor starts
+    FF (Finish-to-Finish): Predecessor must finish before successor finishes
+    SF (Start-to-Finish): Predecessor must start before successor finishes (rare)
+    """
+    FS = "FS"  # Finish-to-Start
+    SS = "SS"  # Start-to-Start
+    FF = "FF"  # Finish-to-Finish
+    SF = "SF"  # Start-to-Finish
+
+
+class TaskDependency(Base):
+    """
+    Represents a dependency relationship between two tasks.
+
+    The predecessor task affects when the successor task can be scheduled,
+    based on the dependency_type. This is used for Gantt chart visualization
+    and date validation.
+    """
+    __tablename__ = "pjctrl_task_dependencies"
+
+    __table_args__ = (
+        UniqueConstraint('predecessor_id', 'successor_id', name='uq_predecessor_successor'),
+    )
+
+    id = Column(String(36), primary_key=True)
+    predecessor_id = Column(
+        String(36),
+        ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    successor_id = Column(
+        String(36),
+        ForeignKey("pjctrl_tasks.id", ondelete="CASCADE"),
+        nullable=False,
+        index=True
+    )
+    dependency_type = Column(
+        Enum("FS", "SS", "FF", "SF", name="dependency_type_enum"),
+        default="FS",
+        nullable=False
+    )
+    lag_days = Column(Integer, default=0, nullable=False)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+
+    # Relationships
+    predecessor = relationship(
+        "Task",
+        foreign_keys=[predecessor_id],
+        back_populates="successors"
+    )
+    successor = relationship(
+        "Task",
+        foreign_keys=[successor_id],
+        back_populates="predecessors"
+    )