feat(spot-check): pace live Edifabric calls at 1.5s apart

Edifabric /v2/x12 endpoints apply a per-second rate limit
(typical: 1 call/sec sustained, 429 if exceeded) on top of the
daily quota. Sequential spot-checks of 10 files tripped it on
back-to-back runs. Insert a 1.5s pause between calls so the
spot-check always lives within the rate budget. This is the
'1-2 s spacing' the plan flagged; the original monolithic
scratch driver skipped it and the verifier noted the gap on
retry.

backend/src/cyclone/rebill/spot_check_validate.py

@@ -74,9 +74,18 @@ def validate_spot_check_files(paths: list[Path]) -> list[dict]:
             This module never silently substitutes a different
             validator.
     """
+    import time
     out: list[dict] = []
-    for p in paths:
+    for i, p in enumerate(paths):
         body = p.read_bytes()
+        # The Edifabric /v2/x12 endpoints apply a per-second rate limit
+        # in addition to the daily quota (typical: 1 call/sec sustained,
+        # 429 if exceeded). Insert a 1.5s pause between calls so 10-file
+        # spot-checks don't trip the limit; this is the value the plan
+        # flagged ("1-2 s spacing") that the original monolithic scratch
+        # driver skipped.
+        if i > 0:
+            time.sleep(1.5)
         try:
             result = validate_edi(body)
         except EdifabricError as exc: