Add luacheck static analysis to CI

[levleontiev]

Summary

Add luacheck as a static analysis step in CI to catch unused variables, undefined globals, shadowed variables, and syntax errors in Lua source files.

Motivation

We already have lua-quality CI step but luacheck provides more comprehensive static analysis. For OpenResty/LuaJIT code, luacheck helps catch common bugs early.

Acceptance Criteria

• luacheck installed in docker/Dockerfile.test (or CI directly via apt/luarocks)
• .luacheckrc config added at repo root, declaring OpenResty globals (ngx, cjson, require, etc.) to avoid false positives
• New lua-quality or separate luacheck step in .github/workflows/ci.yml running luacheck src/fairvisor/
• CI step fails on luacheck errors (not continue-on-error)
• Existing source files pass (fix any warnings found)

Tooling options

• luacheck (https://github.com/lunarmodules/luacheck) — gold standard, widely used, easy CI integration
• selene (https://github.com/Kampfkarren/selene) — Rust-based alternative, faster, more configurable

Recommendation: start with luacheck.

Notes

• The .luacheckrc must whitelist OpenResty-specific globals: ngx, ndk, cjson, cjson_safe, require (already a Lua global)
• Can pin to a specific version for reproducibility