perf: shared http.Transport with tuned connection pooling

Add internal/httpclient package as a singleton RoundTripper used by
all outbound engine requests (search, engines, autocomplete, upstream).

Key Transport settings:
- MaxIdleConnsPerHost = 20  (up from Go default of 2)
- MaxIdleConns = 100
- IdleConnTimeout = 90s
- DialContext timeout = 5s

Previously, the default transport limited each host to 2 idle connections,
forcing a new TCP+TLS handshake on every search for each engine. With
12 engines hitting the same upstream hosts in parallel, connections
were constantly recycled. Now warm connections are reused across all
goroutines and requests.

internal/search/service.go

@@ -18,11 +18,11 @@ package search
 
 import (
 	"context"
-	"net/http"
 	"sync"
 	"time"
 
 	"github.com/metamorphosis-dev/samsa/internal/cache"
+	"github.com/metamorphosis-dev/samsa/internal/httpclient"
 	"github.com/metamorphosis-dev/samsa/internal/config"
 	"github.com/metamorphosis-dev/samsa/internal/contracts"
 	"github.com/metamorphosis-dev/samsa/internal/engines"
@@ -49,7 +49,7 @@ func NewService(cfg ServiceConfig) *Service {
 		timeout = 10 * time.Second
 	}
 
-	httpClient := &http.Client{Timeout: timeout}
+	httpClient := httpclient.NewClient(timeout)
 
 	var up *upstream.Client
 	if cfg.UpstreamURL != "" {