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

backend/app/schemas/common.py

@@ -0,0 +1,18 @@
+"""Shared schema building blocks (pagination meta, error envelope)."""
+from pydantic import BaseModel, ConfigDict
+
+
+class Meta(BaseModel):
+    total: int
+    page: int
+    page_size: int
+
+
+class ErrorBody(BaseModel):
+    error: dict
+
+
+class APIModel(BaseModel):
+    """Base for request schemas — Pydantic v2, permissive on extra fields."""
+
+    model_config = ConfigDict(str_strip_whitespace=True)