Make maximum number of threads used by web server configurable

[DL6ER]

What does this implement/fix?

See title. On second thought, this seems like a meaningful addition. We reduce the default to always leave at least one core unoccupied in case issues like described in #2129 appear (which is still unresolved due to us still waiting for feedback). Still, this PR seems meaningful to avoid this from preventing others to other users as well.

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Related issue or feature (if applicable): N/A

Pull request in docs with documentation (if applicable): N/A

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• The code change is tested and works locally.
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Copilot reviewed 1 out of 6 changed files in this pull request and generated no comments.

Files not reviewed (5)

• src/api/docs/content/specs/config.yaml: Language not supported
• src/config/config.c: Language not supported
• src/config/config.h: Language not supported
• src/webserver/webserver.c: Language not supported
• src/webserver/webserver.h: Language not supported

Comments suppressed due to low confidence (1)

test/pihole.toml:668

• [nitpick] The explanation of the values -1 and -2 might be confusing. Consider rephrasing to: 'The default value of -1 means that the number of threads is automatically determined by the number of online CPU cores (e.g., launching up to 8 threads on 8 cores). A value of -2 means the number of threads is twice the number of CPU cores (e.g., launching up to 16 threads on 8 cores).'

# by the number of online CPU cores (e.g.,launching up to 8 threads on 8 cores). A