feat(duckduckgo): detect anti-bot challenge and surface ErrBlocked

DuckDuckGo intermittently serves a CAPTCHA modal ("Unfortunately, bots
use DuckDuckGo too...") instead of search results. The result selector
matches zero elements on that page, so callers used to get
([]Result{}, nil) — silent empty results that look like "no matches."

Detect the challenge via the BEM class .anomaly-modal__title and return
a typed ErrBlocked so callers can distinguish blocked from no-results
and react (retry, fallback to another engine, surface to user).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

sites/duckduckgo/duckduckgo.go

@@ -2,6 +2,7 @@ package duckduckgo
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"log/slog"
 	"net/url"
@@ -9,6 +10,10 @@ import (
 	"gitea.stevedudenhoeffer.com/steve/go-extractor"
 )
 
+// ErrBlocked is returned when DuckDuckGo serves an anti-bot challenge
+// page instead of search results.
+var ErrBlocked = errors.New("duckduckgo: blocked by anti-bot challenge")
+
 type SafeSearch int
 
 const (