fix: various security fixes (#38406)

Addresses a batch of privately reported security issues, grouped by
area:

- **SSRF** - migration PR-patch/asset fetches, OAuth2 avatar & OpenID
discovery, pull-mirror URL re-validation, and the outbound proxy path.
- **Access-token scope** - prevent scope escalation on token creation;
keep public-only tokens confined (feeds, packages, Actions listings,
star/watch lists, limited/private owners).
- **Access control / disclosure** - go-get default-branch leak, webhook
authorization-header leak, watch clearing on private transitions,
label/attachment scoping.
- **Denial of service** - input bounds for npm dist-tags, Debian control
files, Arch file lists, and SSH keys.

### 📌 Attention for site admins

Not breaking - existing configs keep working - but two changes are worth
a look:

- **New SSRF protection** Outbound requests (migrations, OAuth2 avatars,
OpenID discovery, pull mirrors, proxy path) are now validated against
the allow/block host lists. If your instance legitimately reaches
internal hosts, you may need to add them to
`[security].ALLOWED_HOST_LIST` (and the relevant `ALLOW_LOCALNETWORKS`
settings).
- **Deprecation** `[webhook].ALLOWED_HOST_LIST` is deprecated and will
be removed in a future release. Use `[security].ALLOWED_HOST_LIST`
instead; the old key still works for now.

---------

Co-authored-by: TheFox0x7 <thefox0x7@gmail.com>
Co-authored-by: techknowlogick <techknowlogick@gitea.io>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: Zettat123 <zettat123@gmail.com>

services/migrations/gitlab.go

@@ -94,7 +94,7 @@ type GitlabDownloader struct {
 //	Use either a username/password, personal token entered into the username field, or anonymous/public access
 //	Note: Public access only allows very basic access
 func NewGitlabDownloader(ctx context.Context, baseURL, repoPath, token string) (*GitlabDownloader, error) {
-	gitlabClient, err := gitlab.NewClient(token, gitlab.WithBaseURL(baseURL), gitlab.WithHTTPClient(NewMigrationHTTPClient()))
+	gitlabClient, err := gitlab.NewClient(token, gitlab.WithBaseURL(baseURL), gitlab.WithHTTPClient(newMigrationHTTPClient()))
 	if err != nil {
 		log.Trace("Error logging into gitlab: %v", err)
 		return nil, err
@@ -314,7 +314,7 @@ func (g *GitlabDownloader) convertGitlabRelease(ctx context.Context, rel *gitlab
 		PublisherName:   rel.Author.Username,
 	}
 
-	httpClient := NewMigrationHTTPClient()
+	httpClient := newMigrationHTTPClient()
 
 	for _, asset := range rel.Assets.Links {
 		assetID := asset.ID // Don't optimize this, for closure we need a local variable