Add API endpoint to refresh the registry cache

[rdimitrov]

Summary

When ToolHive is configured with a custom registry API (registry_api_url), thv serve caches the registry data in memory for 1 hour. If the upstream registry changes during that window, API clients (like the UI) see stale data and have no way to ask for fresh data short of restarting the server.

This adds POST /api/v1beta/registry/{name}/refresh so the UI (or any API client) can force a refresh on demand. It does the same thing the CLI's thv registry list --refresh already does: type-asserts the provider to CachedAPIRegistryProvider and calls ForceRefresh().

Closes #5266

Type of change

• Bug fix (non-breaking change which fixes an issue)

Test plan

• Unit tests added in pkg/api/v1/registry_test.go:
  • happy path: GET returns stale data, refresh succeeds, subsequent GET returns fresh data
  • non-default registry name → 404
  • unavailable upstream → 503 with registry_unavailable code
• task lint-fix passes

Does this introduce a user-facing change?

Yes — new API endpoint POST /api/v1beta/registry/{name}/refresh (currently only default is supported). Returns {"status": "refreshed"} on success.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 47.61905% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 68.25%. Comparing base (110b9a7) to head (c283190).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/api/v1/registry.go	47.61%	9 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5268      +/-   ##
==========================================
+ Coverage   68.17%   68.25%   +0.08%     
==========================================
  Files         618      618              
  Lines       63122    63205      +83     
==========================================
+ Hits        43033    43143     +110     
+ Misses      16878    16841      -37     
- Partials     3211     3221      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.