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>

modules/uri/uri.go

@@ -23,13 +23,21 @@ func (e ErrURISchemeNotSupported) Error() string {
 
 // Open open a local file or a remote file
 func Open(uriStr string) (io.ReadCloser, error) {
+	return OpenWithClient(uriStr, http.DefaultClient)
+}
+
+// OpenWithClient opens a local file or a remote file, using the given (non-nil) HTTP client
+// for http/https URLs. Callers that must confine remote access (e.g. to defeat SSRF via
+// redirects) should pass a client whose transport validates the peer at dial time; Open
+// passes http.DefaultClient.
+func OpenWithClient(uriStr string, client *http.Client) (io.ReadCloser, error) {
 	u, err := url.Parse(uriStr)
 	if err != nil {
 		return nil, err
 	}
 	switch strings.ToLower(u.Scheme) {
 	case "http", "https":
-		f, err := http.Get(uriStr)
+		f, err := client.Get(uriStr)
 		if err != nil {
 			return nil, err
 		}