Count hits, misses, fetches and transforms as Prometheus counters (closes #311)

The five counts the cache_stats table held are now Prometheus counters in
memory, in internal/imgcache/counters.go, on the default registry that
/metrics serves. The Increment methods add to them, so counting makes no
database call.

cache_stats, Cache.Stats, Service.Stats, CacheStats and the pending counts
are gone, with their tests. The background goroutine now writes only access
times, and its names say so: StartPendingAccessTimeWrites,
StopPendingAccessTimeWrites and pendingaccesstimes.go.

The counters count for the whole process, so the tests that check how much
they rose do not call t.Parallel. Two stop tests whose write under way was a
hit count now use an access time.

Model: opus-5-5

TODO.md

@@ -30,6 +30,17 @@ P2: security: per-origin rate limiting
 
 # Completed Steps
 
+- 2026-10-26 cache hits, misses, upstream fetches, the bytes those fetches read
+  and transforms are counted as Prometheus counters in memory (closes #311).
+  They are `pixa_cache_hits_total`, `pixa_cache_misses_total`,
+  `pixa_upstream_fetches_total`, `pixa_upstream_fetch_bytes_total` and
+  `pixa_transforms_total`, in `internal/imgcache/counters.go`, on the default
+  registry that `/metrics` serves. The `Increment` methods add to them.
+  `cache_stats`, `Cache.Stats`, `Service.Stats`, `CacheStats` and the pending
+  counts are gone. The background goroutine writes only access times now:
+  `StartPendingAccessTimeWrites` starts it and `StopPendingAccessTimeWrites`
+  stops it, in `pendingaccesstimes.go`. The counters count for the whole
+  process, so the tests that check them do not call `t.Parallel`.
 - 2026-10-21 the schema file declares only the tables and columns pixa writes
   and reads (closes #310): `output_content` and `request_cache` are gone, and so
   are the columns nothing read: the host, path, query, `content_type` and