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>

models/auth/access_token_scope.go

@@ -304,6 +304,36 @@ func (s AccessTokenScope) PublicOnly() (bool, error) {
 	return bitmap.hasScope(AccessTokenScopePublicOnly)
 }
 
+// CanCreateChildScope reports whether a request authenticated by this (parent) scope may mint a token
+// carrying the child scope. It rejects any grantable scope the parent does not hold, closing the
+// scope-escalation path. public-only is a restriction rather than a grantable permission, so it is
+// ignored here (a child may always be public-only); EnforcePublicOnlyFrom handles carrying it down.
+func (s AccessTokenScope) CanCreateChildScope(child AccessTokenScope) (bool, error) {
+	requested := child.StringSlice()
+	scopes := make([]AccessTokenScope, 0, len(requested))
+	for _, sc := range requested {
+		childScope := AccessTokenScope(sc)
+		if childScope == AccessTokenScopePublicOnly {
+			continue
+		}
+		scopes = append(scopes, childScope)
+	}
+	return s.HasScope(scopes...)
+}
+
+// EnforcePublicOnlyFrom adds the public-only restriction to s when the authorizing parent scope is
+// public-only, so a public-only token cannot mint a child token that drops the restriction.
+func (s AccessTokenScope) EnforcePublicOnlyFrom(parent AccessTokenScope) (AccessTokenScope, error) {
+	publicOnly, err := parent.PublicOnly()
+	if err != nil {
+		return "", err
+	}
+	if !publicOnly {
+		return s, nil
+	}
+	return AccessTokenScope(string(s) + "," + string(AccessTokenScopePublicOnly)).Normalize()
+}
+
 // HasScope returns true if the string has the given scope
 func (s AccessTokenScope) HasScope(scopes ...AccessTokenScope) (bool, error) {
 	bitmap, err := s.parse()