feat: add /autocompleter endpoint for search suggestions

Proxies to upstream SearXNG /autocompleter if configured, otherwise
falls back to Wikipedia OpenSearch API. Returns a JSON array of
suggestion strings.

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

internal/autocomplete/service.go

@@ -0,0 +1,124 @@
+package autocomplete
+
+import (
+	"context"
+	"encoding/json"
+	"errors"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+// Service fetches search suggestions from an upstream SearXNG instance
+// or falls back to Wikipedia's OpenSearch API.
+type Service struct {
+	upstreamURL string
+	http       *http.Client
+}
+
+func NewService(upstreamURL string, timeout time.Duration) *Service {
+	if timeout <= 0 {
+		timeout = 5 * time.Second
+	}
+	return &Service{
+		upstreamURL: strings.TrimRight(upstreamURL, "/"),
+		http: &http.Client{Timeout: timeout},
+	}
+}
+
+// Suggestions returns search suggestions for the given query.
+func (s *Service) Suggestions(ctx context.Context, query string) ([]string, error) {
+	if strings.TrimSpace(query) == "" {
+		return nil, nil
+	}
+
+	if s.upstreamURL != "" {
+		return s.upstreamSuggestions(ctx, query)
+	}
+	return s.wikipediaSuggestions(ctx, query)
+}
+
+// upstreamSuggestions proxies to an upstream SearXNG /autocompleter endpoint.
+func (s *Service) upstreamSuggestions(ctx context.Context, query string) ([]string, error) {
+	u := s.upstreamURL + "/autocompleter?" + url.Values{"q": {query}}.Encode()
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Accept", "application/json")
+
+	resp, err := s.http.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, errors.New("upstream autocompleter failed")
+	}
+
+	body, err := io.ReadAll(io.LimitReader(resp.Body, 64*1024))
+	if err != nil {
+		return nil, err
+	}
+
+	// SearXNG /autocompleter returns a plain JSON array of strings.
+	var out []string
+	if err := json.Unmarshal(body, &out); err != nil {
+		return nil, err
+	}
+	return out, nil
+}
+
+// wikipediaSuggestions fetches suggestions from Wikipedia's OpenSearch API.
+func (s *Service) wikipediaSuggestions(ctx context.Context, query string) ([]string, error) {
+	u := "https://en.wikipedia.org/w/api.php?" + url.Values{
+		"action":       {"opensearch"},
+		"format":       {"json"},
+		"formatversion": {"2"},
+		"search":       {query},
+		"namespace":    {"0"},
+		"limit":        {"10"},
+	}.Encode()
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set(
+		"User-Agent",
+		"gosearch-go/0.1 (compatible; +https://github.com/metamorphosis-dev/kafka)",
+	)
+
+	resp, err := s.http.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		return nil, errors.New("wikipedia opensearch failed")
+	}
+
+	body, err := io.ReadAll(io.LimitReader(resp.Body, 64*1024))
+	if err != nil {
+		return nil, err
+	}
+
+	// Wikipedia OpenSearch returns: [query, [suggestions], ...]
+	var data []json.RawMessage
+	if err := json.Unmarshal(body, &data); err != nil {
+		return nil, err
+	}
+	if len(data) < 2 {
+		return nil, nil
+	}
+
+	var suggestions []string
+	if err := json.Unmarshal(data[1], &suggestions); err != nil {
+		return nil, err
+	}
+	return suggestions, nil
+}