chore(ci): run Security Updates weekly instead of daily

The job finishes in well under a minute but is billed a full minute per
run, so ~29 daily runs/month is almost entirely rounding overhead.
Detection cadence is not the bottleneck for these alerts; merge latency
is. Moves to Mondays 03:37 UTC.

Claude-Session: https://claude.ai/code/session_01Xke8d7un3d7fBQcLHrkWr8

Author: albertotb

.github/workflows/security-updates.yml

@@ -2,7 +2,7 @@ name: Security Updates
 
 on:
   schedule:
-    - cron: "30 3 * * *"
+    - cron: "37 3 * * 1"
   workflow_dispatch:
 
 permissions: