Fix missing timeout in Discord health check

[osse101]

This PR replaces the usage of the standard library's http.Get in the Discord bot's health check (internal/discord/health.go) with a properly configured http.Client that includes a timeout. The global http.Get does not set a timeout by default, which could lead to the health check endpoint silently hanging if the backend API server is unresponsive. Using h.bot.Client.Client.Get resolves this issue and aligns with the project's codebase patterns for making external HTTP requests.

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PR created automatically by Jules for task 8953561629141403637 started by @osse101

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