Test metrics auth, CORS preflight, login logging and metrics (closes #79)

New tests only. MetricsAuth on its own answers 401 with a challenge
without credentials or with a wrong username or password, and lets the
configured ones through. A CORS preflight request gets the same
Access-Control-Allow-Origin as a GET. A POST / carrying the signing key
leaves the key out of the request log line, and the login handler's own
log lines leave out the submitted key. The metrics middleware on its own
records a request it served; the router records nothing while no metrics
username is set.

Not tested through the router: the basic auth in front of /metrics and
recording with a metrics username set (#180).

The pinned basicauth-go compares the password in constant time.

Model: opus-5-5

TODO.md

@@ -29,6 +29,21 @@ P2: security: referer blacklist
 
 # Completed Steps
 
+- 2026-10-04 the metrics basic auth, CORS preflight, request logging and
+  metrics recording have tests (closes #79): `MetricsAuth` on its own answers
+  401 with a challenge without credentials or with a wrong username or password
+  and lets the configured ones through; a preflight request gets `*` for any
+  origin when `access_control_allow_origin` is `*` and no
+  `Access-Control-Allow-Origin` from another origin than the configured one; a
+  `POST /` carrying the signing key leaves no trace of it in the request log
+  line, and the login handler's own log lines leave out the submitted key; the
+  metrics middleware on its own records a request it served, and the router
+  records nothing while no metrics username is set. Not tested: that the router
+  puts the basic auth in front of `/metrics` and records requests when a
+  metrics username is set. Only one test per package can set up `/metrics`, and
+  in `internal/server` that is `TestMaintenanceModeKeepsOtherRoutes`, which
+  needs the owner's approval to change; #180 holds it. Tests only; the basic
+  auth library already compares the password in constant time.
 - 2026-10-04 the image route's signature check and error answers are tested
   (closes #76): new tests in `internal/handlers`, with no network, check the
   status and JSON error body for a missing, wrong, unpadded, upper-case or