feat(admin): add fleet statistics and admin read endpoints

Counters (always on when admin is enabled):
- last seen per user and group, written asynchronously after
  admission, throttled to one write per scope per minute per replica
- hourly provider and model attempts, failures and timeouts
- failovers per provider, rejections per hour, and unpriced (402)
  refusals
- cache hits, misses and cost saved
- request counts per target and caller

Opt-in counters under admin.stats (both default off):
- userModel: usage per user and model
- latency: latency histograms per provider and model

All counters ride the existing Redis batches. A test measures three
round trips per request with defaults and with every statistic on. A
cache hit takes two. A model request is no longer counted for failed
failover candidates.

New admin endpoints:
- usage/series and usage/totals
- performance, with percentiles from the histograms
- catalog, with the price billing actually uses and its source
- consumers, with key source, grants and model access
- config, with secrets redacted

The overview gains features and a pricing summary. Usage gains
lastSeen. usage/models gains offset and more detail fields. Reads are
chunked at 1600 keys per round trip and capped at 64000 keys per
request.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

resp.go

@@ -638,12 +638,6 @@ func encodeCommands(cmds [][]string) []byte {
 	return dst
 }
 
-// encodeCommand renders args as a RESP2 command array:
-// "*N\r\n$len\r\narg\r\n..." for each arg.
-func encodeCommand(args []string) string {
-	return string(encodeCommands([][]string{args}))
-}
-
 // decodeReply reads and decodes one RESP2 reply from r. depth is 0 for a
 // top-level reply and depth+1 for each element decodeArray recurses into;
 // an array is only accepted at depth 0 (see the '*' case) — none of this