feat(core): add core module with mixins, services, and background jobs

- Add Model Mixins: TimestampMixin, SoftDeleteMixin, AuditMixin, etc.
- Add Base Services: BaseService, BulkOperationsMixin, QueryOptimizerMixin
- Add Base ViewSets with bulk operations
- Add BackgroundJob model for Celery task tracking
- Add BaseAppCommand for management commands
- Add permissions, pagination, filters, cache, logging
- Migrate tests to factory_boy + faker
- Add CHANGELOG.md
- 297 tests passing

src/apps/core/exceptions.py

@@ -0,0 +1,169 @@
+"""
+Core exceptions for the application.
+
+Provides a hierarchy of business logic exceptions that are automatically
+converted to appropriate API responses by the exception handler.
+"""
+
+from typing import Any
+
+
+class BaseAPIException(Exception):
+    """
+    Base exception for all API-related errors.
+
+    Attributes:
+        message: Human-readable error message
+        code: Machine-readable error code (e.g., 'validation_error')
+        status_code: HTTP status code
+        details: Additional error details (optional)
+    """
+
+    message: str = "An error occurred"
+    code: str = "error"
+    status_code: int = 400
+    details: dict[str, Any] | None = None
+
+    def __init__(
+        self,
+        message: str | None = None,
+        code: str | None = None,
+        details: dict[str, Any] | None = None,
+    ):
+        self.message = message or self.message
+        self.code = code or self.code
+        self.details = details
+        super().__init__(self.message)
+
+    def to_dict(self) -> dict[str, Any]:
+        """Convert exception to dictionary for API response."""
+        result = {
+            "code": self.code,
+            "message": self.message,
+        }
+        if self.details:
+            result["details"] = self.details
+        return result
+
+
+# ============================================================================
+# Client Errors (4xx)
+# ============================================================================
+
+
+class ValidationError(BaseAPIException):
+    """Raised when input data fails validation."""
+
+    message = "Validation error"
+    code = "validation_error"
+    status_code = 400
+
+
+class BadRequestError(BaseAPIException):
+    """Raised when request is malformed or invalid."""
+
+    message = "Bad request"
+    code = "bad_request"
+    status_code = 400
+
+
+class AuthenticationError(BaseAPIException):
+    """Raised when authentication fails."""
+
+    message = "Authentication failed"
+    code = "authentication_error"
+    status_code = 401
+
+
+class PermissionDeniedError(BaseAPIException):
+    """Raised when user lacks required permissions."""
+
+    message = "Permission denied"
+    code = "permission_denied"
+    status_code = 403
+
+
+class NotFoundError(BaseAPIException):
+    """Raised when requested resource is not found."""
+
+    message = "Resource not found"
+    code = "not_found"
+    status_code = 404
+
+
+class ConflictError(BaseAPIException):
+    """Raised when action conflicts with current state."""
+
+    message = "Conflict with current state"
+    code = "conflict"
+    status_code = 409
+
+
+class RateLimitError(BaseAPIException):
+    """Raised when rate limit is exceeded."""
+
+    message = "Rate limit exceeded"
+    code = "rate_limit_exceeded"
+    status_code = 429
+
+
+# ============================================================================
+# Server Errors (5xx)
+# ============================================================================
+
+
+class InternalError(BaseAPIException):
+    """Raised for unexpected internal errors."""
+
+    message = "Internal server error"
+    code = "internal_error"
+    status_code = 500
+
+
+class ServiceUnavailableError(BaseAPIException):
+    """Raised when a dependent service is unavailable."""
+
+    message = "Service temporarily unavailable"
+    code = "service_unavailable"
+    status_code = 503
+
+
+# ============================================================================
+# Business Logic Errors
+# ============================================================================
+
+
+class BusinessLogicError(BaseAPIException):
+    """
+    Base class for business logic errors.
+
+    Use this for domain-specific errors that should return 400/422.
+    """
+
+    message = "Business logic error"
+    code = "business_error"
+    status_code = 400
+
+
+class InvalidStateError(BusinessLogicError):
+    """Raised when entity is in invalid state for requested operation."""
+
+    message = "Invalid state for this operation"
+    code = "invalid_state"
+    status_code = 400
+
+
+class DuplicateError(BusinessLogicError):
+    """Raised when attempting to create a duplicate resource."""
+
+    message = "Resource already exists"
+    code = "duplicate"
+    status_code = 409
+
+
+class QuotaExceededError(BusinessLogicError):
+    """Raised when a resource quota is exceeded."""
+
+    message = "Quota exceeded"
+    code = "quota_exceeded"
+    status_code = 400