service: no per-service usage stats are recorded

[edwin-zvs]

Nothing is recorded per service. There are no counters anywhere in the service module, so an operator running a service has no way to answer "is it being used, is it working, and what is it costing".

The design calls for, per service: requests by outcome per channel, sessions created and currently active, queue and turn latency, tokens by harness and model, and rate-limit rejections.

Two notes on sequencing:

• The rate-limit rejection counter only becomes meaningful once budgets exist (service: per-service budgets (rate, concurrency, tokens) are not enforced #1132), but the rest stands alone and is useful immediately.
• Counters must survive daemon restart, alongside definitions and key-to-session maps. ServiceShared already persists state atomically to data_dir/services/<name>.json and is a per-service singleton for the daemon's lifetime, so it is the natural owner — but note the invariant that it must never be recreated on reload, or counters reset silently.

Surfacing is a second question: the TUI service view and the web UI both currently show definition state only.