feat: add OpenSearch XML endpoint

- Serve /opensearch.xml with configurable base URL
- Browsers can now add gosearch as a search engine from the address bar
- Configurable via [server] base_url or BASE_URL env var
- XML template embedded in the binary via go:embed
- Added base_url to config.example.toml

internal/config/config.go

@@ -22,6 +22,7 @@ type Config struct {
 type ServerConfig struct {
 	Port        int    `toml:"port"`
 	HTTPTimeout string `toml:"http_timeout"`
+	BaseURL     string `toml:"base_url"` // Public URL for OpenSearch XML (e.g. "https://search.example.com")
 }
 
 type UpstreamConfig struct {
@@ -158,6 +159,9 @@ func applyEnvOverrides(cfg *Config) {
 	if v := os.Getenv("RATE_LIMIT_CLEANUP_INTERVAL"); v != "" {
 		cfg.RateLimit.CleanupInterval = v
 	}
+	if v := os.Getenv("BASE_URL"); v != "" {
+		cfg.Server.BaseURL = v
+	}
 }
 
 // HTTPTimeout parses the configured timeout string into a time.Duration.