Refactor backend to SQLModel; reset schema; add Company OS endpoints

backend/app/schemas/hr.py

@@ -0,0 +1,24 @@
+from __future__ import annotations
+
+from sqlmodel import SQLModel
+
+
+class HeadcountRequestCreate(SQLModel):
+    department_id: int
+    requested_by_manager_id: int
+    role_title: str
+    employee_type: str
+    quantity: int = 1
+    justification: str | None = None
+
+
+class HeadcountRequestUpdate(SQLModel):
+    status: str | None = None
+    justification: str | None = None
+
+
+class EmploymentActionCreate(SQLModel):
+    employee_id: int
+    issued_by_employee_id: int
+    action_type: str
+    notes: str | None = None