Stop waiting for count writes after a request's deadline (closes #224)

TestService_Get_ReturnsByItsDeadline failed on a busy host because its
request, past its deadline, still waited for its miss count to be
written, a database write with no deadline; in pixad that write waits
for the one database connection every request shares. The hit, miss,
upstream fetch and transform counts are now each written in a goroutine
of their own, which the request waits for only until its deadline; a
count not written by then is written after it returns. Shutdown waits
for those writes within ShutdownTimeout and reports any left unfinished.
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 +30,21 @@ P2: security: per-IP rate limiting on the image routes
 
 # Completed Steps
 
+- 2026-10-08 a request past its deadline no longer waits for a database write
+  (closes #224). On a busy host, `TestService_Get_ReturnsByItsDeadline` failed
+  because its request returned over a second after its deadline: it was still
+  waiting for its miss count to be written to the database, a write with no
+  deadline, which in pixad waits for the one database connection that every
+  request shares. The hit, miss, upstream fetch and transform counts are now
+  each written in a goroutine of their own, which the request waits for only
+  until its deadline; a count not written by then is written after the request
+  returns. On shutdown pixad waits for those writes within the same 5 seconds as
+  for the images being processed, and exits with 1 when some are unfinished. 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