Count a late hit, and report no totals on a disabled cache (closes #56)

A hit is now counted with the request context detached from its
cancellation, as the miss already is, so a hit served after the client
left or the request timed out still moves the hit count. On a disabled
disk cache, Stats reports no items and no size, whatever rows an earlier
run left in the database; the size already followed that rule.

Model: opus-5-5

TODO.md

@@ -46,12 +46,14 @@ exhaustion
   counts the cached source images and processed variants (`source_content`
   plus `variant_content`) and takes their size from `Cache.UsageBytes`,
   instead of reading `request_cache` and `output_content`, which nothing
-  writes; those two tables are left in the schema. A miss is counted after
-  it is served or fails, even when the request context has ended by then,
-  with the bytes it read from upstream, so `upstream_fetch_count` and
-  `upstream_fetch_bytes` move, including for an upstream body that fails
-  partway or a fetched source that then fails the magic byte check;
-  `transform_count` counts each image the image processor transcodes.
+  writes; those two tables are left in the schema; a disabled disk cache
+  reports no items and no size. A hit is counted even when the request
+  context has ended. A miss is counted after it is served or fails, also
+  when the request context has ended by then, with the bytes it read from
+  upstream, so `upstream_fetch_count` and `upstream_fetch_bytes` move,
+  including for an upstream body that fails partway or a fetched source
+  that then fails the magic byte check; `transform_count` counts each image
+  the image processor transcodes.
 - 2026-09-28 strip metadata from processed images (closes #82): every output is
   exported with govips' `StripMetadata`, so it carries no EXIF, XMP, IPTC or ICC
   profile; the image is first turned upright with `AutoRotate` (before sizes are