Corefile: drop explicit cache 30, use plugin default (3600)

The cache 30 directive in the (common) snippet was clamping
authoritative TTLs to 30s max — every record HE pulled showed TTL≈5
because the cache plugin intercepts responses regardless of source
(auto plugin authoritative answers AND forward plugin resolver answers).

Switching to bare 'cache' uses the plugin's 3600s default, which
preserves our source TTLs: most records at 300s, _dmarc/dkim/SRV at
3600s, wildcards at 60s.

Corefile

@@ -27,7 +27,11 @@
     forward . 1.1.1.1 1.0.0.1 9.9.9.9 {
         max_concurrent 1000
     }
-    cache 30
+    # Use default cap (3600). Earlier `cache 30` clamped authoritative
+    # TTLs too aggressively — every record HE pulled showed TTL≈5 because
+    # the cache plugin sits in the (common) plugin chain and clamps any
+    # response passing through, not just forwarded-resolver answers.
+    cache
     errors
     log
     loop