test(uploads): pin the deployed idle bound and its unavailable warning (#56)

Gatekeeper review of PR #63 (comment 15139):
- the 60 s default is what production runs (cmd/tapir never sets
  UploadIdleTimeout) and no test covered it; 1 ns, 60 ms or 24 h all left
  the suite green. Pinned via export_test.go, plus a socket test with the
  timeout unset that accepts a body with 100 ms gaps (60 ms mutant red).
- the "upload idle bound unavailable" warning, the only signal that the
  control did not apply, is now asserted (removing it goes red).
- the deadline clear on io.EOF was redundant (net/http resets the
  deadline before its background read) and is removed; ADR-033 text
  corrected (the part can end before the body, so "cleared on clean
  EOF" did not hold).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/web/uploads.go

@@ -172,15 +172,10 @@ type idleDeadlineBody struct {
 
 func (b *idleDeadlineBody) Read(p []byte) (int, error) {
 	_ = b.rc.SetReadDeadline(time.Now().Add(b.idle))
-	n, err := b.ReadCloser.Read(p)
-	if errors.Is(err, io.EOF) {
-		// The body is complete: disarm, so the deadline cannot fire on the server's own
-		// background read while the handler is still probing the file. On any other error
-		// it stays armed: net/http drains an unread remainder before replying, and that
-		// drain must stay bounded too.
-		_ = b.rc.SetReadDeadline(time.Time{})
-	}
-	return n, err
+	// Never cleared here: net/http clears the read deadline itself before its background
+	// read once the handler has the body, and an armed deadline after an error also bounds
+	// net/http's drain of an unread remainder before it replies.
+	return b.ReadCloser.Read(p)
 }
 
 // abandoned reports, logs at INFO and answers 499, because the client has already gone: its request