Let an operator actually reach the browser limits

[davidmckayv]

Two problems with the browser cap #90 added, both found by driving it against the real container
rather than by reading the diff.

The settings were unreachable. COMPUTER_MAX_BROWSERS and COMPUTER_BROWSER_IDLE_MS are read by
profiles.ts and were declared nowhere, so COMPUTER_MAX_BROWSERS=1 docker compose up did nothing:
the container never saw the variable. A limit an operator cannot set is not a limit.

Declaring them exposed the real bug. An unset variable in a compose file arrives as an empty
string rather than as absent, so process.env.X ?? 8 never fires and Number("") is zero. Every
deployment that had not set a cap would have got a cap of zero: every browser closed the moment it
opened, which reads as a broken computer rather than a misconfigured one. Anything that is not a
positive number now falls back, because "I typed this wrong" and "I did not set it" both mean the
default.

This is the second time today an empty-string environment value has been the bug worth catching, the
first being OPENBOT_SINGLE_USER. Worth remembering when adding a setting.

Proof

Against the real container, rebuilt:

• COMPUTER_MAX_BROWSERS=1: opening beta's browser closed alpha's, logged
  computer-browser-closed with the cap on running browsers was reached, and left the Bot that
  just asked running.
• alpha came straight back on its next request, because the profile is on the volume. Closing one
  costs a relaunch and nothing else.
• Variable empty: three Bots all kept their browsers, so the default applied rather than zero.

Two more tests, one of which states the failure as the behaviour it would have caused.
bun run test:ci: 1010 tests, 0 fail.

Where it runs

• New state that outlives a request? None.
• What happens on the second replica? Each computer container enforces its own limits, which is
  the right scope: the memory being bounded is that container's.
• Anything serialised? No.
• Anything fanned out to a browser? No.
• New listener, port, or schedule? None beyond the idle sweep Put shared state where every replica can see it, and bound what grows #90 added, which is unrefed.