🧹 [Code Health] Refactor do_POST in serve.py for better routing structure

[Ch3fUlrich]

🎯 What: The do_POST method in lib/linux/serve.py was overly long because it handled routing and execution logic for all API endpoints in one block. I extracted the logic for /api/claude/snapshot, /api/config, and /api/install into dedicated methods (_post_claude_snapshot, _post_config, and _post_install).

💡 Why: Splitting this logic improves maintainability and readability, as each endpoint handler is now self-contained rather than combined into a single, massive branching function.

✅ Verification: I ran python3 -m py_compile lib/linux/serve.py to ensure there were no syntax errors. I also ran the full test suite (bash tests/run-tests.sh) to guarantee that all endpoints and their behavior continue to function correctly.

✨ Result: A much cleaner do_POST routing function that is easier to read and extend.

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PR created automatically by Jules for task 15747632028358794963 started by @Ch3fUlrich

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