revert: block app.listen() until tenant registry is ready (#4062)

[bokelley]

Summary

Reverts #4062. Awaiting tenant warmup before app.listen() made Fly's deploy-time health-check timeout (300s) fire before the new machines came healthy — the deploy itself failed instead of just the smoke. Worse outcome than the original problem (smoke flake but production healthy).

What was tried

1. fix(training-agent): eager tenant registry init at server boot #4060 — eager init at module load. Init started during construction but didn't gate listen, so smoke still raced. Net effect: 5 more failed-but-non-blocking deploys.
2. fix(training-agent): block app.listen() until tenant registry is ready #4062 — captured warmup() and awaited it before app.listen(). Made Fly's health check time out before the listener bound. Deploy failed with Unrecoverable error: timeout reached waiting for health checks to pass.

Tenant registry init is taking >300s on a fresh Fly machine for reasons that aren't visible from the workflow logs (need flyctl logs --app adcp-docs for the boot-time stdout). Until we know why, blocking listen is the wrong tool.

Where to go from here

Options:

• Smoke-side fix: extend the post-deploy smoke's retry budget from 16s to ~120s. Real bugs (fix(training-agent): wire Postgres task registry — production hotfix #3854, fix(training-agent): drop noopJwksValidator production guard #3869 class) still surface (deterministic, won't pass even after 120s); slow init no longer false-fails. Smallest blast radius.
• Diagnose the slow init: pull flyctl logs --app adcp-docs during a deploy, find the actual long pole. Could be DNS, JWKS validation against an unreachable host, framework-server build, etc.
• Lazy → background-eager hybrid: kick off init at server boot (as fix(training-agent): eager tenant registry init at server boot #4060 did) but also write a sentinel file when complete. Smoke probes the sentinel before probing tenant routes.

For now, restoring main to the #4060 state — at least the deploy completes, even if the smoke still flakes.

Test plan

• Typecheck clean
• Tenant tests pass (no functional change vs. fix(training-agent): eager tenant registry init at server boot #4060)
• After merge: deploy succeeds (Fly health check passes, machines come up)
• Smoke step fails (expected, same flake as before fix(training-agent): block app.listen() until tenant registry is ready #4062)

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