feat: migrate from env vars to config.toml

- Add internal/config package with TOML parsing (BurntSushi/toml)
- Create config.example.toml documenting all settings
- Update main.go to load config via -config flag (default: config.toml)
- Environment variables remain as fallback overrides for backward compat
- Config file values are used as defaults; env vars override when set
- Add comprehensive tests for file loading, defaults, and env overrides
- Add config.toml to .gitignore (secrets stay local)

go.mod

@@ -3,6 +3,7 @@ module github.com/ashie/gosearch
 go 1.25.0
 
 require (
+	github.com/BurntSushi/toml v1.5.0
 	github.com/PuerkitoBio/goquery v1.12.0 // indirect
 	github.com/andybalholm/cascadia v1.3.3 // indirect
 	golang.org/x/net v0.52.0 // indirect