feat: single-line progress indicator during crawl and ext-link check

Uses \r to overwrite the same terminal line — no scrolling.
Crawler exposes on_progress callback; checker.check_external_links
gets an optional on_progress parameter.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scanner/crawler.py

@@ -91,6 +91,7 @@ class Crawler:
         self.headers = {"User-Agent": cfg["user_agent"]}
         self.session = requests.Session()
         self.session.headers.update(self.headers)
+        self.on_progress: object = None  # callable(fetched: int, queued: int)
 
     def crawl(self) -> dict:
         """
@@ -176,6 +177,9 @@ class Crawler:
                         if link_url not in seen and link_url not in queue:
                             queue.append(link_url)
 
+                if self.on_progress:
+                    self.on_progress(len(pages), len(queue))
+
             except requests.exceptions.RequestException as exc:
                 errors.append({"url": url, "error": str(exc)})
                 logger.warning("Error crawling %s: %s", url, exc)