feat: exclude_paths — skip dynamic areas (RSS feeds, news archives) from scan

Adds crawl.exclude_paths config option (list of URL path prefixes).
Matching URLs are excluded at two levels:
- Crawler: not fetched at all (saves HTTP requests)
- Differ: filtered from both baseline and current before comparison,
  so legacy baseline entries under excluded paths don't appear as
  "missing URLs" after the option is added retroactively

Example config:
  crawl:
    exclude_paths: ["/rss/", "/feed/", "/news/archiv/"]

8 new tests (176 total).

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

tests/test_crawler.py

@@ -71,3 +71,25 @@ class TestShouldCrawl:
 
     def test_utm_query_param_rejected(self):
         assert not should_crawl("https://example.com/?utm_source=x", "example.com", SKIP_EXT)
+
+    def test_excluded_path_prefix_rejected(self):
+        assert not should_crawl(
+            "https://example.com/rss/feed.xml", "example.com", SKIP_EXT,
+            exclude_paths=["/rss/"],
+        )
+
+    def test_excluded_path_subpage_rejected(self):
+        assert not should_crawl(
+            "https://example.com/news/archiv/2024/artikel", "example.com", SKIP_EXT,
+            exclude_paths=["/news/archiv/"],
+        )
+
+    def test_similar_path_not_excluded(self):
+        """'/rss-link/' must not be excluded when only '/rss/' is configured."""
+        assert should_crawl(
+            "https://example.com/rss-link/", "example.com", SKIP_EXT,
+            exclude_paths=["/rss/"],
+        )
+
+    def test_no_exclude_paths_unchanged(self):
+        assert should_crawl("https://example.com/page/", "example.com", SKIP_EXT, exclude_paths=[])