feat: Add fast startup fetch and background caching

- Add  for quick startup using first N feeds
- Add background thread for full fetch and caching
- Update  to use fast fetch
- Update docs and skills

tests/test_fetch.py

@@ -31,12 +31,12 @@ class TestFetchFeed:
 
     @patch("engine.fetch.urllib.request.urlopen")
     def test_fetch_network_error(self, mock_urlopen):
-        """Network error returns None."""
+        """Network error returns tuple with None feed."""
         mock_urlopen.side_effect = Exception("Network error")
 
-        result = fetch_feed("http://example.com/feed")
+        url, feed = fetch_feed("http://example.com/feed")
 
-        assert result is None
+        assert feed is None
 
 
 class TestFetchAll:
@@ -54,7 +54,7 @@ class TestFetchAll:
             {"title": "Headline 1", "published_parsed": (2024, 1, 1, 12, 0, 0)},
             {"title": "Headline 2", "updated_parsed": (2024, 1, 2, 12, 0, 0)},
         ]
-        mock_fetch_feed.return_value = mock_feed
+        mock_fetch_feed.return_value = ("http://example.com", mock_feed)
         mock_skip.return_value = False
         mock_strip.side_effect = lambda x: x
 
@@ -67,7 +67,7 @@ class TestFetchAll:
     @patch("engine.fetch.boot_ln")
     def test_fetch_all_feed_error(self, mock_boot, mock_fetch_feed):
         """Feed error increments failed count."""
-        mock_fetch_feed.return_value = None
+        mock_fetch_feed.return_value = ("http://example.com", None)
 
         items, linked, failed = fetch_all()
 
@@ -87,7 +87,7 @@ class TestFetchAll:
             {"title": "Sports scores"},
             {"title": "Valid headline"},
         ]
-        mock_fetch_feed.return_value = mock_feed
+        mock_fetch_feed.return_value = ("http://example.com", mock_feed)
         mock_skip.side_effect = lambda x: x == "Sports scores"
         mock_strip.side_effect = lambda x: x