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)

internal/config/config_test.go

@@ -0,0 +1,143 @@
+package config
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+)
+
+func TestLoadDefaults(t *testing.T) {
+	cfg, err := Load("/nonexistent/config.toml")
+	if err != nil {
+		t.Fatalf("Load with missing file should return defaults: %v", err)
+	}
+	if cfg.Server.Port != 8080 {
+		t.Errorf("expected default port 8080, got %d", cfg.Server.Port)
+	}
+	if len(cfg.Engines.LocalPorted) != 5 {
+		t.Errorf("expected 5 default engines, got %d", len(cfg.Engines.LocalPorted))
+	}
+}
+
+func TestLoadFromFile(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.toml")
+	content := `
+[server]
+port = 9090
+http_timeout = "30s"
+
+[upstream]
+url = "http://localhost:8888"
+
+[engines]
+local_ported = ["wikipedia", "braveapi"]
+
+[engines.brave]
+api_key = "test-key"
+access_token = "secret"
+
+[engines.qwant]
+category = "web"
+results_per_page = 5
+`
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatalf("Load failed: %v", err)
+	}
+
+	if cfg.Server.Port != 9090 {
+		t.Errorf("expected port 9090, got %d", cfg.Server.Port)
+	}
+	if cfg.Server.HTTPTimeout != "30s" {
+		t.Errorf("expected http_timeout 30s, got %s", cfg.Server.HTTPTimeout)
+	}
+	if cfg.Upstream.URL != "http://localhost:8888" {
+		t.Errorf("expected upstream URL, got %s", cfg.Upstream.URL)
+	}
+	if len(cfg.Engines.LocalPorted) != 2 {
+		t.Errorf("expected 2 engines, got %d", len(cfg.Engines.LocalPorted))
+	}
+	if cfg.Engines.Brave.APIKey != "test-key" {
+		t.Errorf("expected brave api_key, got %s", cfg.Engines.Brave.APIKey)
+	}
+	if cfg.Engines.Brave.AccessToken != "secret" {
+		t.Errorf("expected brave access_token, got %s", cfg.Engines.Brave.AccessToken)
+	}
+	if cfg.Engines.Qwant.Category != "web" {
+		t.Errorf("expected qwant category web, got %s", cfg.Engines.Qwant.Category)
+	}
+	if cfg.Engines.Qwant.ResultsPerPage != 5 {
+		t.Errorf("expected qwant results_per_page 5, got %d", cfg.Engines.Qwant.ResultsPerPage)
+	}
+}
+
+func TestEnvOverrides(t *testing.T) {
+	t.Setenv("PORT", "3000")
+	t.Setenv("HTTP_TIMEOUT", "5s")
+	t.Setenv("UPSTREAM_SEARXNG_URL", "http://env:9999")
+	t.Setenv("BRAVE_API_KEY", "env-key")
+	t.Setenv("LOCAL_PORTED_ENGINES", "wikipedia,arxiv")
+
+	cfg, err := Load("/nonexistent/config.toml")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if cfg.Server.Port != 3000 {
+		t.Errorf("expected env port 3000, got %d", cfg.Server.Port)
+	}
+	if cfg.Server.HTTPTimeout != "5s" {
+		t.Errorf("expected env timeout 5s, got %s", cfg.Server.HTTPTimeout)
+	}
+	if cfg.Upstream.URL != "http://env:9999" {
+		t.Errorf("expected env upstream URL, got %s", cfg.Upstream.URL)
+	}
+	if cfg.Engines.Brave.APIKey != "env-key" {
+		t.Errorf("expected env brave key, got %s", cfg.Engines.Brave.APIKey)
+	}
+	if len(cfg.Engines.LocalPorted) != 2 {
+		t.Errorf("expected 2 env engines, got %d", len(cfg.Engines.LocalPorted))
+	}
+}
+
+func TestConfigFileWinsOverDefaults(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.toml")
+	content := `
+[server]
+port = 7070
+`
+	if err := os.WriteFile(path, []byte(content), 0644); err != nil {
+		t.Fatal(err)
+	}
+
+	t.Setenv("PORT", "3000") // env should override config
+
+	cfg, err := Load(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Env overrides the config file
+	if cfg.Server.Port != 3000 {
+		t.Errorf("env should override config: expected 3000, got %d", cfg.Server.Port)
+	}
+}
+
+func TestHTTPTimeout(t *testing.T) {
+	cfg := &Config{Server: ServerConfig{HTTPTimeout: "5s"}}
+	if d := cfg.HTTPTimeout(); d.String() != "5s" {
+		t.Errorf("expected 5s, got %s", d)
+	}
+
+	// Invalid falls back to 10s
+	cfg.Server.HTTPTimeout = "not-a-duration"
+	if d := cfg.HTTPTimeout(); d.String() != "10s" {
+		t.Errorf("expected fallback 10s, got %s", d)
+	}
+}