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

config.example.toml

@@ -17,7 +17,7 @@ url = ""
 [engines]
 # Comma-separated list of engines to execute locally in Go (env: LOCAL_PORTED_ENGINES)
 # Engines not listed here will be proxied to upstream SearXNG.
-local_ported = ["wikipedia", "arxiv", "crossref", "braveapi", "qwant"]
+local_ported = ["wikipedia", "arxiv", "crossref", "braveapi", "qwant", "duckduckgo", "github", "reddit", "bing"]
 
 [engines.brave]
 # Brave Search API key (env: BRAVE_API_KEY)