tasks: tick P2 completion + Playwright sweep screenshots

P2R-09/10/11/12/13/14, P2-16/17/18 all marked done. Acceptance line
for Windows hosts annotated as 'compile-verified, untested in CI'.

_diag/p2-completion-sweep/ holds the dashboard + host-detail +
schedules + sources + repo + source-group-edit screenshots from a
clean sweep against :8080. Zero console errors throughout.

announce_test.go: rate-limit + global-cap subtests dropped t.Parallel
to avoid racing on the package-level tunables under -race.

internal/server/http/announce_test.go

@@ -111,10 +111,9 @@ func TestAnnounceHostnameCollisionFlag(t *testing.T) {
 }
 
 func TestAnnounceRateLimit(t *testing.T) {
-	t.Parallel()
+	// Not t.Parallel — mutates the package-level announceMaxPerMin
+	// var, which would otherwise race other announce tests.
 	_, url, _ := newTestServerWithHub(t)
-	// Lower the limit for the duration of this test (the limiter is
-	// per-server-instance so we don't disturb parallel tests).
 	prev := announceMaxPerMin
 	announceMaxPerMin = 2
 	t.Cleanup(func() { announceMaxPerMin = prev })
@@ -137,7 +136,7 @@ func TestAnnounceRateLimit(t *testing.T) {
 }
 
 func TestAnnounceGlobalCap(t *testing.T) {
-	t.Parallel()
+	// Not t.Parallel — mutates the package-level announceGlobalCap.
 	_, url, st := newTestServerWithHub(t)
 	prev := announceGlobalCap
 	announceGlobalCap = 1