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/checker.py

@@ -123,6 +123,7 @@ def check_external_links(
     urls: list[str],
     timeout: int = 10,
     delay: float = 0.3,
+    on_progress: object = None,
 ) -> dict[str, dict]:
     """
     Check a list of URLs for reachability via HEAD (fallback: GET).
@@ -158,6 +159,8 @@ def check_external_links(
                 results[url] = {"status": resp.status_code, "final_url": resp.url, "error": None}
             except requests.exceptions.RequestException as exc:
                 results[url] = {"status": None, "final_url": None, "error": str(exc)}
+            if on_progress:
+                on_progress(i + 1, len(urls))
     finally:
         session.close()