Clamp the HTTP drain by the tail-hook reserve (closes #170)

The HTTP drain at shutdown waited up to ShutdownTimeout regardless of how much of the stop budget earlier hooks had used, so a slow archive sweeper or retention reaper could eat the reserve the hooks after the server need, and the database close was skipped. The drain now waits at most the shorter of ShutdownTimeout and what is left of the budget less TailHookReserve, as the Sentry flush already does. The reserve is documented as derived from the two timeouts. Tests cover earlier hooks having spent part of the budget, on a clock that host speed cannot move, and pin that a drain on the full budget gets all of ShutdownTimeout.

Model: opus-5-5

internal/server/export_test.go

@@ -1,6 +1,8 @@
 package server
 
 import (
+	"context"
+	"log/slog"
 	"net/http"
 	"testing"
 
@@ -37,6 +39,14 @@ func SentryClientOptionsForTest(
 	return sentryClientOptions(dsn, release)
 }
 
+// CleanShutdownForTest runs the server's stop hook, cleanShutdown,
+// against hs: a server the test started itself, so it can hold a
+// request open across the drain. Sentry is off.
+func CleanShutdownForTest(ctx context.Context, hs *http.Server) {
+	s := &Server{log: slog.New(slog.DiscardHandler), httpServer: hs}
+	s.cleanShutdown(ctx)
+}
+
 // newServerForTest builds a Server through New, as the application
 // does, on a lifecycle that is never started: the hooks New adds to
 // it never run, so nothing listens.