fix(selfhost): retry the /ready probe in selfhost-post-update-check.sh instead of a single attempt

[JSONbored]

Summary

• `docker compose up -d` returns as soon as the container starts, well before the app inside has finished booting and bound its port — a single immediate `curl -sf` reliably false-failed on a completely normal deploy. Hit twice today deploying today's other fixes.
• The `gittensory` service's own Docker healthcheck (`docker-compose.yml`) already documents and tolerates exactly this: `start_period: 60s` "tolerates the Postgres cold start." `selfhost-post-update-check.sh` had no equivalent tolerance.
• Replaced the single probe with a bounded retry loop (default 45 × 2s = 90s, over the documented 60s `start_period`), configurable via `SELFHOST_READY_RETRIES`/`SELFHOST_READY_RETRY_DELAY_SECONDS`.

Closes #5085

Scope

• The PR title follows `type(scope): short summary` Conventional Commit format.
• This PR is focused and does not mix unrelated backend, UI, MCP, docs, dependency, and deploy changes.
• This follows `CONTRIBUTING.md`.
• I linked a currently open issue this PR resolves (`Closes selfhost-post-update-check.sh false-fails on a normal deploy: single /ready probe, no retry #5085`).

Validation

• `git diff --check`
• `bash -n` + `shellcheck` clean (only a pre-existing, unrelated SC1091 info note on the sourced lib file)
• `npm run typecheck`
• Ran the 3 existing test files that reference this script (`docs-selfhost-update-rollback.test.ts`, `docs-selfhost-git-deploy-hygiene.test.ts`, `selfhost-update-script.test.ts`) — all pass unchanged.
• Functionally verified the retry loop against local mock servers: one that returns 503 twice then 200 (succeeds on the 3rd attempt), and one that never responds (exhausts retries, exits non-zero with a clear message).
• New or changed behavior has unit/integration tests for new branches, fallback paths, and sanitizer boundaries — this is a bash deploy script under `scripts/`, which Codecov's patch gate does not measure (only `src/` is measured); verified via the functional checks above instead of a vitest unit test, matching how the pre-existing tests for this file (stub-based, doc-hygiene) already treat it.

Safety

• No secrets, wallet details, hotkeys, coldkeys, user PATs, private keys, raw trust scores, private rankings, or private maintainer evidence are exposed.
• Public GitHub text stays sanitized, low-noise, and does not imply compensation guarantees or optimization tactics.
• Auth, cookie, CORS, GitHub App, Cloudflare, or session changes include negative-path tests. (N/A.)
• API/OpenAPI/MCP behavior is updated and tested where needed. (N/A.)
• UI changes use live API data or real empty/error/loading states. (N/A.)
• Visible UI changes include a `UI Evidence` section. (N/A — no UI changed.)
• Public docs/changelogs are updated where needed. (N/A — the operator-facing docs already reference this script by name only, no specific retry-count claim to keep in sync.)

[superagent-security]

Superagent didn't find any vulnerabilities or security issues in this PR.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.20%. Comparing base (f22516c) to head (60242f3).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5086   +/-   ##
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  Coverage   94.20%   94.20%           
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  Files         470      470           
  Lines       39722    39722           
  Branches    14493    14493           
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  Hits        37421    37421           
  Misses       1645     1645           
  Partials      656      656           

Flag	Coverage Δ
shard-1	46.42% <ø> (ø)
shard-2	33.58% <ø> (ø)
shard-3	31.34% <ø> (ø)
shard-4	32.80% <ø> (ø)
shard-5	33.54% <ø> (ø)
shard-6	44.90% <ø> (ø)

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