Export cache, eviction, upstream and transform metrics (closes #85)

New pixa_ metrics, none with labels:
- source cache hits and misses, counted once per variant made or checked
- cache usage and limit gauges; usage is set each time eviction reads it
- eviction count and bytes, from the size_bytes the deleted row returns
- upstream fetch duration histogram and fetch error counter
- transform duration histogram and an in-progress gauge, both including
  the wait for a processing slot

Removing a damaged file during reconciliation is not counted as an
eviction. README lists every metric; the metrics config comment now says
the pixa_ metrics are measured whether or not /metrics is served.

Model: opus-5-5

internal/providers/providers.go

@@ -35,7 +35,7 @@ import (
 
 // Constructors returns the constructors of the parts this package builds,
 // for fx.Provide. The fx graph must also provide the prometheus.Registerer
-// the cache and the image service register their counters on.
+// the cache and the image service register their metrics on.
 func Constructors() []any {
 	return []any{
 		newKeys,