fix: prevent SSH timeout stacking in MirrorSQL

[jakebromberg]

Summary

• Each sshInstance() call in MirrorSQL created a new 5-minute setTimeout without clearing the previous one, causing dangling timeouts to stack on rapid repeated calls.
• Stores the timeout handle as a static class property and calls clearTimeout on the existing handle before setting a new one.
• Adds unit tests verifying only one timeout is active after multiple calls, and that superseded timeouts don't trigger premature disposal.

Test plan

• New test: tests/unit/middleware/legacy/ssh-timeout.test.ts — confirms clearTimeout is called on prior handles and that only the latest timeout fires dispose().
• Tests fail before fix, pass after fix.
• Existing unit tests still pass (npm run test:unit).

Fixes #36

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