feat(backend): add FastAPI backend — procurement, inventory domain, Casdoor auth, PM arrival endpoints

backend/app/utils/errors.py

@@ -0,0 +1,51 @@
+"""Application error type + standardized error envelope.
+
+All errors are returned as: {"error": {"code", "message", "field"}}
+(contracts: expert-opinion-p0.md «General Shared Contracts»).
+"""
+
+
+class AppError(Exception):
+    """Base error carrying an HTTP status and the contract error envelope."""
+
+    def __init__(self, status_code: int, code: str, message: str, field: str | None = None):
+        super().__init__(message)
+        self.status_code = status_code
+        self.code = code
+        self.message = message
+        self.field = field
+
+    @property
+    def body(self) -> dict:
+        error = {"code": self.code, "message": self.message}
+        if self.field is not None:
+            error["field"] = self.field
+        return {"error": error}
+
+
+class ValidationError(AppError):
+    """400 VALIDATION_ERROR — bad input (types, business validation)."""
+
+    def __init__(self, message: str, field: str | None = None):
+        super().__init__(400, "VALIDATION_ERROR", message, field)
+
+
+class NotFoundError(AppError):
+    """404 NOT_FOUND."""
+
+    def __init__(self, message: str = "رکورد یافت نشد", field: str | None = None):
+        super().__init__(404, "NOT_FOUND", message, field)
+
+
+class ConflictError(AppError):
+    """409 CONFLICT — duplicates, invalid state transitions, protected deletes."""
+
+    def __init__(self, message: str, field: str | None = None):
+        super().__init__(409, "CONFLICT", message, field)
+
+
+class ForbiddenError(AppError):
+    """403 FORBIDDEN — role / ownership checks."""
+
+    def __init__(self, message: str, field: str | None = None):
+        super().__init__(403, "FORBIDDEN", message, field)