Keep counts not written by the request's deadline in memory (closes #224)

TestService_Get_ReturnsByItsDeadline failed on a busy host because its
request, past its deadline, still waited to write its miss count, a write
with no deadline; in pixad that write waits for the one database
connection every request shares. Each count write now keeps the request's
deadline but not its cancellation. A count not written by then is kept in
memory, where Stats includes it, and one goroutine of the cache writes
those counts one UPDATE at a time, and once more at shutdown before the
database closes. The test phase also runs go test with -parallel 4: on a
busy host, as many tests at once as there are CPUs wait so long to be
scheduled that a timed request can fail.

Model: opus-5-5

TODO.md

@@ -30,6 +30,19 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 a request past its deadline no longer waits for the database to
+  count it (closes #224). On a busy host, `TestService_Get_ReturnsByItsDeadline`
+  failed because its request, past its deadline, still waited for its miss count
+  to be written, a write with no deadline, which in pixad waits for the one
+  database connection every request shares. Each count write now keeps the
+  request's deadline but not its cancellation. A count not written by then is
+  kept in memory, where `Stats` sees it, and one goroutine of the cache writes
+  those counts, one write at a time, and once more at shutdown before the
+  database closes. The test phase of the `Dockerfile` also runs at most 4 tests
+  of a package at once (`go test -parallel 4`), where it ran as many as the host
+  has CPUs: on a busy host they then wait so long to be scheduled that a test
+  that times a request can fail. `-p`, how many packages are tested at once, is
+  unchanged, as capping it also slows compiling.
 - 2026-10-08 every output format is saved with settings pixa sets on purpose
   (closes #232): each format has its own govips export, as JPEG XL does, in
   place of govips' generic `Export`, which sent libvips a zero for some settings