feat: add DuckDuckGo, GitHub, Reddit, and Bing engines

- DuckDuckGo: scrapes Lite HTML endpoint for results
  - Language-aware region mapping (de→de-de, ja→jp-jp, etc.)
  - HTML parser extracts result links and snippets from DDG Lite markup
  - Shared html_helpers.go with extractAttr, stripHTML, htmlUnescape

- GitHub: uses public Search API (repos, sorted by stars)
  - No auth required (10 req/min unauthenticated)
  - Shows stars, language, topics, last updated date
  - Paginated via GitHub's page parameter

- Reddit: uses public JSON search API
  - Respects safesearch (skips over_18 posts)
  - Shows subreddit, score, comment count
  - Links self-posts to the thread URL

- Bing: scrapes web search HTML (b_algo containers)
  - Extracts titles, URLs, and snippets from Bing's result markup
  - Handles Bing's tracking URL encoding

- Updated factory, config defaults, and config.example.toml
- Full test suite: unit tests for all engines, HTML parsing tests,
  region mapping tests, live request tests (skipped in short mode)

9 engines total: wikipedia, arxiv, crossref, braveapi, qwant,
duckduckgo, github, reddit, bing

internal/config/config.go

@@ -92,7 +92,7 @@ func defaultConfig() *Config {
 		},
 		Upstream: UpstreamConfig{},
 		Engines: EnginesConfig{
-			LocalPorted: []string{"wikipedia", "arxiv", "crossref", "braveapi", "qwant"},
+			LocalPorted: []string{"wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing"},
 			Qwant: QwantConfig{
 				Category:       "web-lite",
 				ResultsPerPage: 10,

internal/config/config_test.go

@@ -14,8 +14,8 @@ func TestLoadDefaults(t *testing.T) {
 	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))
+	if len(cfg.Engines.LocalPorted) != 9 {
+		t.Errorf("expected 9 default engines, got %d", len(cfg.Engines.LocalPorted))
 	}
 }