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/httpapi/handlers.go

@@ -1,7 +1,10 @@
 package httpapi
 
 import (
+	"context"
+	"encoding/json"
 	"net/http"
+	"strings"
 
 	"github.com/metamorphosis-dev/kafka/internal/contracts"
 	"github.com/metamorphosis-dev/kafka/internal/search"
@@ -9,11 +12,15 @@ import (
 )
 
 type Handler struct {
-	searchSvc *search.Service
+	searchSvc          *search.Service
+	autocompleteSvc    func(ctx context.Context, query string) ([]string, error)
 }
 
-func NewHandler(searchSvc *search.Service) *Handler {
-	return &Handler{searchSvc: searchSvc}
+func NewHandler(searchSvc *search.Service, autocompleteSuggestions func(ctx context.Context, query string) ([]string, error)) *Handler {
+	return &Handler{
+		searchSvc:       searchSvc,
+		autocompleteSvc: autocompleteSuggestions,
+	}
 }
 
 func (h *Handler) Healthz(w http.ResponseWriter, r *http.Request) {
@@ -96,3 +103,21 @@ func (h *Handler) Search(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusInternalServerError)
 	}
 }
+
+// Autocompleter returns search suggestions for the given query.
+func (h *Handler) Autocompleter(w http.ResponseWriter, r *http.Request) {
+	query := strings.TrimSpace(r.FormValue("q"))
+	if query == "" {
+		w.WriteHeader(http.StatusBadRequest)
+		return
+	}
+
+	suggestions, err := h.autocompleteSvc(r.Context(), query)
+	if err != nil {
+		// Return empty list on error rather than an error status.
+		suggestions = []string{}
+	}
+
+	w.Header().Set("Content-Type", "application/json; charset=utf-8")
+	_ = json.NewEncoder(w).Encode(suggestions)
+}