over-engineering audit: remove dead code, speculative config, duplicate queues

- delete embedded community resolver snapshot (6,760 lines) + EmbeddedServers +
  DNS_SERVERS=fresh; curated resolvers are the default. Simplify Warm (no
  sampling).
- one generic internal/sample.Queue[T] replaces three near-identical
  random-sample queues (probe, pipeline, discover).
- delete unreachable index.listOpenSearch (OpenSearch is never listable).
- delete classifyCloudFront (its match is always discarded, !ResourceSpecific).
- delete probe.Probe wrapper + Describe; tests use ProbeDetailed.
- delete store.Pool(), model.SourceDNSAlias, discover.QueueLen.
- drop dead config/no-op knobs: pipeline DirectWorkers, discover MaxPerDomain,
  discover S3/GCS/DO flags, BUCKET_SCAN_DO.
~-7,000 lines (mostly the resolver snapshot), 0 deps removed, tests green.

.env.example

@@ -11,7 +11,6 @@ DATABASE_URL=postgres://ctcloud:change-me-strong@postgres:5432/ctcloud?sslmode=d
 # DATABASE_URL=postgres://user:[email protected]:5432/ctcloud?sslmode=verify-full
 
 # DNS_SERVERS unset = ~20 curated public resolvers. Options:
-#   DNS_SERVERS=fresh              curated + compiled-in Fresh-Resolvers (sampled)
 #   DNS_SERVERS=system             use the OS resolver
 #   DNS_SERVERS=1.1.1.1,8.8.8.8    your own comma-separated list
 # DNS_SERVERS=system
@@ -34,4 +33,3 @@ BUCKET_SCAN_ENABLED=true
 BUCKET_SCAN_QPS=50
 BUCKET_SCAN_WORKERS=16
 BUCKET_SCAN_QUEUE=65536
-BUCKET_SCAN_DO=false