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/webhook/general.go

@@ -404,20 +404,17 @@ func ToHook(repoLink string, w *webhook_model.Webhook) (*api.Hook, error) {
 		config["color"] = s.Color
 	}
 
-	authorizationHeader, err := w.HeaderAuthorization()
-	if err != nil {
-		return nil, err
-	}
-
 	return &api.Hook{
-		ID:                  w.ID,
-		Name:                w.Name,
-		Type:                w.Type,
-		URL:                 fmt.Sprintf("%s/settings/hooks/%d", repoLink, w.ID),
-		Active:              w.IsActive,
-		Config:              config,
-		Events:              w.EventsArray(),
-		AuthorizationHeader: authorizationHeader,
+		ID:     w.ID,
+		Name:   w.Name,
+		Type:   w.Type,
+		URL:    fmt.Sprintf("%s/settings/hooks/%d", repoLink, w.ID),
+		Active: w.IsActive,
+		Config: config,
+		Events: w.EventsArray(),
+		// the stored authorization header is a secret and must never be returned by the API,
+		// consistent with the webhook secret which is also omitted from the response
+		AuthorizationHeader: "",
 		Updated:             w.UpdatedUnix.AsTime(),
 		Created:             w.CreatedUnix.AsTime(),
 		BranchFilter:        w.BranchFilter,