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/httpclient/dial.go

@@ -0,0 +1,30 @@
+// samsa — a privacy-respecting metasearch engine
+// Copyright (C) 2026-present metamorphosis-dev
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program. If not, see <https://www.gnu.org/licenses/>.
+
+package httpclient
+
+import (
+	"context"
+	"net"
+	"time"
+)
+
+// dialWithTimeout returns a DialContext function for http.Transport that
+// respects the given connection timeout.
+func dialWithTimeout(timeout time.Duration) func(context.Context, string, string) (net.Conn, error) {
+	d := &net.Dialer{Timeout: timeout}
+	return d.DialContext
+}