fix(avatar): use sha256 and inline the federated avatar lookup (#38843)

- Hash emails with sha256. Gravatar moved to sha256, and both it and
libravatar.org serve the same image for either hash.
- Drop `strk.kbt.io/projects/go/libravatar` for a 46 line inline SRV
lookup. It could not bound or cancel its DNS query and panicked on an
unexpected resolver error. The replacement carries the request context
and a 3s timeout.
- Fix federated avatars querying DNS for every avatar on every render.
`loadAvatarSetting` compared a cache field that was never assigned, so
each call rebuilt the resolver and dropped its cache. That cache is
gone, both settings are read where they are used.
- Migration 348 recreates `email_hash` with a 64 char hash column and a
`hash_type` column, so a later algorithm change can tell old rows apart.
The MD5 rows are unreachable and their `UNIQUE` email index would reject
the SHA256 replacements.
- Fix a re-saved avatar form replacing an uploaded avatar with a random
one.
- Remove the `duoshuo` `GRAVATAR_SOURCE` alias, that service shut down
in 2017.
- Remove dead i18n key.

Fixes: https://github.com/go-gitea/gitea/issues/34284
Fixes: https://github.com/go-gitea/gitea/issues/28110
Docs: https://gitea.com/gitea/docs/pulls/499
Signed-off-by: silverwind <me@silverwind.io>

modules/avatar/federated.go

@@ -0,0 +1,55 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package avatar
+
+import (
+	"context"
+	"errors"
+	"net"
+	"strconv"
+	"strings"
+	"time"
+
+	"gitea.dev/modules/cache"
+)
+
+// LookupFederatedHost returns the avatar host from the email domain's SRV record. https://wiki.libravatar.org/api/
+func LookupFederatedHost(ctx context.Context, email string, secure bool) string {
+	at := strings.LastIndexByte(email, '@')
+	if at < 0 {
+		return ""
+	}
+	domain := strings.ToLower(email[at+1:])
+
+	service, defaultPort := "avatars", uint16(80)
+	if secure {
+		service, defaultPort = "avatars-sec", 443
+	}
+
+	host, _ := cache.GetString(cache.SafeCacheKey("Avatar:SRV:"+service, domain), func() (string, error) {
+		lookupCtx, cancel := context.WithTimeout(ctx, 3*time.Second) // a slow resolver must not stall the request
+		defer cancel()
+
+		// LookupSRV already sorts by priority and randomizes by weight (RFC 2782)
+		_, records, err := net.DefaultResolver.LookupSRV(lookupCtx, service, "tcp", domain)
+		var dnsErr *net.DNSError
+		if err != nil && !(errors.As(err, &dnsErr) && dnsErr.IsNotFound) {
+			return "", err // a timeout or a cancelled request must not cache as "no record"
+		}
+		if err != nil || len(records) == 0 {
+			return "", nil
+		}
+		return srvHost(records[0], defaultPort), nil
+	})
+	return host
+}
+
+// a target of "." means the service is unavailable (RFC 2782)
+func srvHost(record *net.SRV, defaultPort uint16) string {
+	target := strings.TrimSuffix(record.Target, ".")
+	if target == "" || record.Port == defaultPort {
+		return target
+	}
+	return net.JoinHostPort(target, strconv.Itoa(int(record.Port)))
+}