feat(extractors): add LinkedIn + Instagram with profile-to-posts fan-out

3 social-network extractors that work entirely without auth, using
public embed/preview endpoints + Instagram's own SEO-facing API:

- linkedin_post:      /embed/feed/update/{urn} returns full body,
                      author, image, OG tags. Accepts both the urn:li:share
                      and urn:li:activity URN forms plus the pretty
                      /posts/{slug}-{id}-{suffix} URLs.

- instagram_post:     /p/{shortcode}/embed/captioned/ returns the full
                      caption, username, thumbnail. Same endpoint serves
                      reels and IGTV, kind correctly classified.

- instagram_profile:  /api/v1/users/web_profile_info/?username=X with the
                      x-ig-app-id header (Instagram's public web-app id,
                      sent by their own JS bundle). Returns the full
                      profile + the 12 most recent posts with shortcodes,
                      kinds, like/comment counts, thumbnails, and caption
                      previews. Falls back to OG-tag scraping of the
                      public HTML if the API ever 401/403s.

The IG profile output is shaped so callers can fan out cleanly:
  for p in profile.recent_posts:
      scrape('instagram_post', p.url)
giving you 'whole profile + every recent post' in one loop. End-to-end
tested against ticketswave: 1 profile call + 12 post calls in ~3.5s.
Pagination beyond 12 posts requires authenticated cookies and is left
for the cloud where we can stash a session.

Infrastructure change: added FetchClient::fetch_with_headers so
extractors can satisfy site-specific request headers (here x-ig-app-id;
later github_pr will use this for Authorization, etc.) without polluting
the global FetchConfig.headers map. Same retry semantics as fetch().

Catalog now exposes 17 extractors via /v1/extractors. Total unit tests
across the module: 47 passing. Clippy clean. Fmt clean.

Live test on the maintainer's example URLs:
- LinkedIn post (urn:li:share:7452618582213144577): 'Orc Dev' / full body
  / shipper.club link / CDN image extracted in 250ms.
- Instagram post (DT-RICMjeK5): 835-char Slovak caption, ticketswave
  username, thumbnail. 200ms.
- Instagram profile (ticketswave): 18,473 followers (exact, not
  rounded), is_verified=True, is_business=True, biography with emojis,
  12 recent posts with shortcodes + kinds + likes. 400ms.

Out of scope for this wave (require infra we don't have):
- linkedin_profile: returns 999 to all bot UAs, needs OAuth
- facebook_post / facebook_page: content is JS-loaded, needs cloud Chrome
- facebook_profile (personal): not publicly accessible by design

Cargo.lock

@@ -3245,6 +3245,7 @@ dependencies = [
  "http",
  "quick-xml 0.37.5",
  "rand 0.8.5",
+ "regex",
  "serde",
  "serde_json",
  "tempfile",