feat: make model update checks manual configerable

[fl0rianr]

Summary

Adds control over automatic model update checks and introduces an explicit manual update-check command.

Changes

• Add auto_check_model_updates configuration option, enabled by default for backward compatibility
• Skip the startup model update check when the option is disabled
• Add lemonade check-updates for manually checking downloaded models
• Add POST /v1/models/check-updates
• Keep manual model downloads available when automatic checks are disabled
• Respect offline=true for all update checks
• Prevent concurrent startup and manual update checks
• Clear stale update flags after a successful up-to-date response
• Document the new configuration option, CLI command, and API endpoint

Why

Some users need Lemonade to avoid unsolicited network requests during startup without enabling full offline mode, which would also prevent intentional model downloads.

This change separates automatic update checks from explicit user-triggered network actions.

Closes #2608

[bitgamma]

@fl0rianr why is there a .patch file in the commit? I think something went wrong

[fl0rianr]

@bitgamma thanks for catching this one - should have seen the slip since it's actually a concentrated change. Removed exactly this patch. Might have a look on it again?