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, as fields of the cache. NewCache makes them and registers them on
the registry it is given. pixad gives it Prometheus' default registry,
which /metrics serves; each test gives it a new one. The Increment methods
add to their cache's counters, 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 two stop tests
whose write under way was a count now use access times.

Model: opus-5-5

TODO.md

@@ -30,6 +30,18 @@ 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`, fields of the cache. `NewCache` makes them and
+  registers them on the registry it is given (`internal/imgcache/counters.go`).
+  pixad gives it Prometheus' default registry, which `/metrics` serves; each
+  test gives it a new one. The `Increment` methods add to their cache's
+  counters. `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`.
 - 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