feat(client): check broker health before issuance

[lidel]

Problem

A node without a certificate starts ACME issuance even when the registration broker is unreachable (offline network, firewall, outage). Every attempt is doomed: certmagic retries the full flow with backoff for up to 30 days, filling logs with ERRORs that no amount of retrying can resolve.

Fix

• before first-time issuance, startCertManagement waits until the broker answers GET /v1/health with HTTP 204 (15s probe timeout)
• while the check fails, the client logs one ERROR and re-checks after max(1h, min(Retry-After, 24h)); issuance starts automatically once the broker recovers
• probe exported as client.CheckBrokerHealth with the client.HealthCheckPath constant

One cheap GET per interval replaces full ACME attempts while the broker is down. Nodes with a certificate in storage are unaffected; renewal behavior is out of scope.

Merging cuts release v0.10.0 (version.json and changelog are included). Downstream wiring is validated in ipfs/kubo#11397, whose CI is green against this PR's feature commit.

[codecov]

Codecov Report

❌ Patch coverage is 75.43860% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 69.17%. Comparing base (d629320) to head (254b386).

Files with missing lines	Patch %	Lines
client/health.go	74.46%	11 Missing and 1 partial ⚠️
client/acme.go	80.00%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   68.70%   69.17%   +0.46%     
==========================================
  Files          21       22       +1     
  Lines        1713     1765      +52     
==========================================
+ Hits         1177     1221      +44     
- Misses        416      425       +9     
+ Partials      120      119       -1     

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[github-actions]

Suggested tag: v0.10.0

Comparing to: v0.9.1 (diff)

gorelease says:

# summary
v0.10.0 is a valid semantic version for this release.

gocompat says:

HEAD is now at d629320 fix(client): keep registration on one LB backend (#90)
Switched to branch 'main'
Your branch is up to date with 'origin/main'.

Cutting a Release (and modifying code files)

This PR is modifying both version.json and code files (not markdown, YAML, TOML or lock files).
The Release Checker is not able to analyse files that are not checked in to main. This might cause the above analysis to be inaccurate.
Please consider performing all the code changes in a separate PR before cutting the release.

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