Make the resolver pure DNS: JSON access log on stdout instead of a log store

The resolver kept its own query-log store (a bbolt ring, a live ring with SSE
fanout, rolling per-domain counters and dashboard time-series) for the console.
In a multi-node deployment each pod held a different slice of that, so every
view disagreed with itself, and the cost sat on the query path. Charts belong
in Grafana and log search in the log collector, so the store is gone.

internal/accesslog writes one JSON line per notable query to stdout. The default
is what was blocked and what failed (blocked, redirected, SERVFAIL, dropped,
refused); log_cached and sample_rate add ordinary answers for a debugging
session. Lines go through a bounded buffer to a single writer goroutine and are
dropped, and counted in fancydns_access_log_dropped_total, when the stream
cannot keep up, so a stalled log pipe can never stall a query. The config key is
access_log (the old qlog block is ignored, like any unknown key).

The metrics gain what the dashboard used to compute: fancydns_group_queries_total
by ACL group and outcome and fancydns_responses_total by RCODE. Removed with the
store: /api/dashboard, /api/logs, /api/logs/stream, /api/logs/clear,
/api/cache/top, the series rings, and the cache's per-name hit table, which took
a mutex and built a string on every cache hit.

Tests that read the old log now read the access-log lines; the contract test
drops the removed endpoints. The web console still calls them and is changed in
the next commit.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

deploy/config.example.yaml

@@ -161,20 +161,18 @@ acl:
         cidrs: ["192.168.91.0/24"]
       policy: deny
 
-qlog:
+# One JSON line on stdout per query worth looking at, for whatever collects the
+# pod's logs (Loki, a Grafana agent, `kubectl logs`). Nothing is stored here;
+# the metrics count everything. The default is blocked queries and failures.
+access_log:
   enabled: true
-  path: data/queries.db
-  max_entries: 200000
-  flush_interval: 1s
-  # 1.0 logs every query. Lower it on a busy instance: the per-domain
-  # counters are what the dashboard actually needs.
-  sample_rate: 1.0
-  log_blocked: true
-  log_cached: false
-  log_errors: true
-  live_buffer: 2000
-  top_domains: 5000
-  top_ttl: 5m
+  log_blocked: true   # blocklist hits and redirects
+  log_errors: true    # SERVFAIL, drops, ACL refusals
+  log_cached: false   # also cache hits
+  # Fraction of ordinary answers to log as well: 0 none, 1 all. For a short
+  # debugging session; leave at 0 otherwise.
+  sample_rate: 0
+  buffer: 4096        # lines that may wait for the log stream before dropping
 
 cluster:
   enabled: false