fix(resources): add expires_at sentinel comment; move pytest import to module level

circuitforge_core/resources/models.py

@@ -13,7 +13,7 @@ class VRAMLease:
     mb_granted: int
     holder_service: str
     priority: int
-    expires_at: float
+    expires_at: float  # unix timestamp; 0.0 = no expiry
 
     @classmethod
     def create(

tests/test_resources/test_models.py

@@ -1,4 +1,5 @@
 import time
+import pytest
 from circuitforge_core.resources.models import VRAMLease, GpuInfo, NodeInfo
 
 
@@ -27,7 +28,6 @@ def test_vram_lease_create_no_ttl_has_zero_expiry():
 def test_vram_lease_is_immutable():
     lease = VRAMLease.create(gpu_id=0, node_id="heimdall", mb=1024,
                               service="snipe", priority=2)
-    import pytest
     with pytest.raises((AttributeError, TypeError)):
         lease.mb_granted = 999  # type: ignore