feat: implement audit trail module

- Backend (FastAPI):
  - AuditLog and AuditAlert models with Alembic migration
  - AuditService with SHA-256 checksum for log integrity
  - AuditMiddleware for request metadata extraction (IP, user_agent)
  - Integrated audit logging into Task, Project, Blocker APIs
  - Query API with filtering, pagination, CSV export
  - Integrity verification endpoint
  - Sensitive operation alerts with acknowledgement

- Frontend (React + Vite):
  - Admin AuditPage with filters and export
  - ResourceHistory component for change tracking
  - Audit service for API calls

- Testing:
  - 15 tests covering service and API endpoints

- OpenSpec:
  - add-audit-trail change archived

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/audit_log.py

@@ -0,0 +1,77 @@
+import uuid
+from sqlalchemy import Column, String, Text, DateTime, ForeignKey, Enum, Index, JSON
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+import enum
+
+
+class AuditAction(str, enum.Enum):
+    CREATE = "create"
+    UPDATE = "update"
+    DELETE = "delete"
+    RESTORE = "restore"
+    LOGIN = "login"
+    LOGOUT = "logout"
+
+
+class SensitivityLevel(str, enum.Enum):
+    LOW = "low"
+    MEDIUM = "medium"
+    HIGH = "high"
+    CRITICAL = "critical"
+
+
+# Event type to sensitivity level mapping
+EVENT_SENSITIVITY = {
+    "task.create": SensitivityLevel.LOW,
+    "task.update": SensitivityLevel.LOW,
+    "task.delete": SensitivityLevel.MEDIUM,
+    "task.assign": SensitivityLevel.LOW,
+    "task.blocker": SensitivityLevel.MEDIUM,
+    "project.create": SensitivityLevel.MEDIUM,
+    "project.update": SensitivityLevel.MEDIUM,
+    "project.delete": SensitivityLevel.HIGH,
+    "user.login": SensitivityLevel.LOW,
+    "user.logout": SensitivityLevel.LOW,
+    "user.permission_change": SensitivityLevel.CRITICAL,
+    "attachment.upload": SensitivityLevel.LOW,
+    "attachment.download": SensitivityLevel.LOW,
+    "attachment.delete": SensitivityLevel.MEDIUM,
+}
+
+# Events that should trigger alerts
+ALERT_EVENTS = {"project.delete", "user.permission_change"}
+
+
+class AuditLog(Base):
+    __tablename__ = "pjctrl_audit_logs"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    event_type = Column(String(50), nullable=False)
+    resource_type = Column(String(50), nullable=False)
+    resource_id = Column(String(36), nullable=True)
+    user_id = Column(String(36), ForeignKey("pjctrl_users.id", ondelete="SET NULL"), nullable=True)
+    action = Column(
+        Enum("create", "update", "delete", "restore", "login", "logout", name="audit_action_enum"),
+        nullable=False
+    )
+    changes = Column(JSON, nullable=True)
+    request_metadata = Column(JSON, nullable=True)
+    sensitivity_level = Column(
+        Enum("low", "medium", "high", "critical", name="sensitivity_level_enum"),
+        default="low",
+        nullable=False
+    )
+    checksum = Column(String(64), nullable=False)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+
+    # Relationships
+    user = relationship("User", foreign_keys=[user_id])
+    alerts = relationship("AuditAlert", back_populates="audit_log", cascade="all, delete-orphan")
+
+    __table_args__ = (
+        Index("idx_audit_user", "user_id", "created_at"),
+        Index("idx_audit_resource", "resource_type", "resource_id", "created_at"),
+        Index("idx_audit_time", "created_at"),
+    )