Validate user and role names instead of munging binding names

Rancher requires global role binding names to be RFC 1123 labels. The
per-user binding name lowercased the key and replaced only "/", so
usernames containing ".", "_" or "@" still produced an invalid name and
failed at apply, and hyphenated names could collide (foo-bar + baz vs
foo + bar-baz) into one object.

Add validation blocks on var.users requiring the username and every
referenced role name to be RFC 1123 labels, so non-compliant input fails
the plan with an actionable message. The binding name is then built from
the two parts directly. A precondition rejects the remaining hyphen
ambiguity at plan time rather than as a mid-apply conflict. Document the
constraint in the users schema section of the README. The group binding
path is unchanged.

modules/rancher/variables.tf

@@ -51,6 +51,28 @@ variable "users" {
     global_role_bindings = optional(list(string), [])
   }))
   default = {}
+
+  # Rancher names each global role binding as an RFC 1123 label and rejects
+  # anything else at apply time. The binding name is built from the username and
+  # the role name verbatim, so both must already be compliant — fail the plan
+  # with a clear message instead of munging the input and hoping.
+  validation {
+    condition = alltrue([
+      for username in keys(var.users) :
+      can(regex("^[a-z0-9]([a-z0-9-]*[a-z0-9])?$", username))
+    ])
+    error_message = "Usernames must be RFC 1123 labels: lowercase alphanumerics and '-', starting and ending alphanumeric (no '.', '_', '@' or uppercase). Rename config/users/<username>.yaml to a compliant username."
+  }
+
+  validation {
+    condition = alltrue(flatten([
+      for user in values(var.users) : [
+        for role in user.global_role_bindings :
+        can(regex("^[a-z0-9]([a-z0-9-]*[a-z0-9])?$", role))
+      ]
+    ]))
+    error_message = "Values in global_role_bindings must be RFC 1123 labels: lowercase alphanumerics and '-', starting and ending alphanumeric."
+  }
 }
 
 # Custom Rancher global roles (config/roles/<name>.yaml). Map key is the role