added Validation rules

backend/src/middleware/Validation/Employee.md

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+## Employee Management
+
+### \[CREATE/UPDATE\] employee
+* All employee operations require authentication
+* Password changes require current password + new password
+* Only authenticated users can create/update employees
+
+### \[ACTION: delete employee\]
+* Requires authentication
+* Server validates permissions before deletion
+
+### \[ACTION: update availability\]
+* Requires employee ID and plan ID
+* Availability updates must include valid preference levels
+* Only authenticated users can update availabilities
+
+### \[ACTION: update last login\]
+* Requires employee ID
+* Fails silently if update fails (logs error but doesn't block user)
+
+## Employee
+
+### \[CREATE\] Employee
+* `firstname` must not be empty
+* `lastname` must not be empty
+* `password` must be at least 6 characters (in create mode)
+* `employeeType` must be selected
+* Contract type validation:
+  * `manager`, `apprentice` => `contractType` = flexible
+  * `guest` => `contractType` = undefined/NONE
+  * `personell` => `contractType` = small || large
+
+### \[UPDATE\] Employee profile
+* `firstname` must not be empty
+* `lastname` must not be empty
+* Only the employee themselves or admins can update
+
+### \[UPDATE\] Employee password
+* `newPassword` must be at least 6 characters
+* `newPassword` must match `confirmPassword`
+* For admin password reset: no `currentPassword` required
+* For self-password change: `currentPassword` required
+
+### \[UPDATE\] Employee roles
+* Only users with role 'admin' can modify roles
+* At least one employee must maintain 'admin' role
+* Users cannot remove their own admin role
+
+### \[UPDATE\] Employee availability
+* Only active employees can set availability
+* Contract type requirements:
+  * `small` contract: minimum 2 available shifts (preference level 1 or 2)
+  * `large` contract: minimum 3 available shifts (preference level 1 or 2)
+  * `flexible` contract: no minimum requirement
+* Availability can only be set for valid shift patterns in selected plan
+* `shiftId` must be valid and exist in the current plan
+
+### \[ACTION: delete\] Employee
+* Only users with role 'admin' can delete employees
+* Cannot delete yourself
+* Cannot delete the last admin user
+* User confirmation required before deletion
+
+### \[ACTION: edit\] Employee
+* Admins can edit all employees
+* Maintenance users can edit non-admin employees or themselves
+* Regular users can only edit themselves