fix(ssrf): apply ssrf-safe transport to TitleExtractor

The image proxy got an ssrfSafeTransport in commit aca0cff3 that resolves
DNS first, blocks any IP in private/reserved CIDRs, then dials by IP to
defeat DNS rebinding. The TitleExtractor used to construct comments'
PostTitle from Locator.URL — a user-supplied field — was missed by that
fix and kept using http.DefaultTransport. The hostname allowlist there
checks the parsed URL host but never the IP it resolves to, so a domain
suffix-matching an allowed host (or 127.0.0.1 itself when AllowedHosts
is empty) reaches the metadata service or any other internal endpoint.

The same gosec rule (G704) was excluded globally in .golangci.yml as part
of aca0cff3, so this gap was not caught by the linter either.

Extract the transport into a new safehttp package so it lives in one
place and can be reused, then pass safehttp.Transport() into the
TitleExtractor's http.Client at construction (cmd/server.go). The image
proxy switches to safehttp.Transport() too — same behaviour, no longer
duplicated.

Reproduction in title_test.go uses the production-style client to hit
an httptest.Server (always 127.0.0.1) and asserts the dialer refuses
even though "127.0.0.1" is in the allowed-domains list. A control case
shows the same setup without safehttp.Transport returns the page —
making the original vulnerability explicit.

backend/app/cmd/server.go

@@ -39,6 +39,7 @@ import (
 	"github.com/umputun/remark42/backend/app/providers"
 	"github.com/umputun/remark42/backend/app/rest/api"
 	"github.com/umputun/remark42/backend/app/rest/proxy"
+	"github.com/umputun/remark42/backend/app/safehttp"
 	"github.com/umputun/remark42/backend/app/store"
 	"github.com/umputun/remark42/backend/app/store/admin"
 	"github.com/umputun/remark42/backend/app/store/engine"
@@ -621,7 +622,7 @@ func (s *ServerCommand) newServerApp(ctx context.Context) (*serverApp, error) {
 		MaxVotes:               s.MaxVotes,
 		PositiveScore:          s.PositiveScore,
 		ImageService:           imageService,
-		TitleExtractor:         service.NewTitleExtractor(http.Client{Timeout: time.Second * 5}, s.getAllowedDomains()),
+		TitleExtractor:         service.NewTitleExtractor(http.Client{Timeout: time.Second * 5, Transport: safehttp.Transport()}, s.getAllowedDomains()),
 		RestrictedWordsMatcher: service.NewRestrictedWordsMatcher(service.StaticRestrictedWordsLister{Words: s.RestrictedWords}),
 	}
 	dataService.RestrictSameIPVotes.Enabled = s.RestrictVoteIP