feat(extractors): add vertical extractors module + first 6 verticals

New extractors module returns site-specific typed JSON instead of
generic markdown. Each extractor:
- declares a URL pattern via matches()
- fetches from the site's official JSON API where one exists
- returns a typed serde_json::Value with documented field names
- exposes an INFO struct that powers the /v1/extractors catalog

First 6 verticals shipped, all hitting public JSON APIs (no HTML
scraping, zero antibot risk):

- reddit       → www.reddit.com/*/.json
- hackernews   → hn.algolia.com/api/v1/items/{id} (full thread in one call)
- github_repo  → api.github.com/repos/{owner}/{repo}
- pypi         → pypi.org/pypi/{name}/json
- npm          → registry.npmjs.org/{name} + downloads/point/last-week
- huggingface_model → huggingface.co/api/models/{owner}/{name}

Server-side routes added:
- POST /v1/scrape/{vertical}  explicit per-vertical extraction
- GET  /v1/extractors         catalog (name, label, description, url_patterns)

The dispatcher validates that URL matches the requested vertical
before running, so users get "URL doesn't match the X extractor"
instead of opaque parse failures inside the extractor.

17 unit tests cover URL matching + path parsing for each vertical.
Live tests against canonical URLs (rust-lang/rust, requests pypi,
react npm, whisper-large-v3 hf, item 8863 hn, an r/micro_saas post)
all return correct typed JSON in 100-300ms. Sample sizes: github
863B, npm 700B, pypi 1.7KB, hf 3.2KB, hn 38KB (full comment tree).

Marketing positioning: Firecrawl charges 5 credits per /extract call
and you write the schema. Webclaw returns the same JSON in 1 credit
per /scrape/{vertical} call with hand-written deterministic
extractors per site.

crates/webclaw-server/src/main.rs

@@ -79,10 +79,15 @@ async fn main() -> anyhow::Result<()> {
 
     let v1 = Router::new()
         .route("/scrape", post(routes::scrape::scrape))
+        .route(
+            "/scrape/{vertical}",
+            post(routes::structured::scrape_vertical),
+        )
         .route("/crawl", post(routes::crawl::crawl))
         .route("/map", post(routes::map::map))
         .route("/batch", post(routes::batch::batch))
         .route("/extract", post(routes::extract::extract))
+        .route("/extractors", get(routes::structured::list_extractors))
         .route("/summarize", post(routes::summarize::summarize_route))
         .route("/diff", post(routes::diff::diff_route))
         .route("/brand", post(routes::brand::brand))