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>

tests/integration/issue_label_scope_test.go

@@ -0,0 +1,34 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package integration
+
+import (
+	"net/http"
+	"testing"
+
+	"gitea.dev/tests"
+)
+
+// TestUpdateIssueLabelForeignLabel verifies that the web issue-label action rejects a
+// label owned by a different repo/org with 404 (indistinguishable from a nonexistent
+// id), closing the cross-repo label enumeration oracle also on the web side.
+func TestUpdateIssueLabelForeignLabel(t *testing.T) {
+	defer tests.PrepareTestEnv(t)()
+
+	sess := loginUser(t, "user2")
+
+	// label 3 belongs to org3 — foreign to user2/repo1 (issue 1); must be 404, not a 500 oracle
+	req := NewRequestWithValues(t, "POST", "/user2/repo1/issues/labels?issue_ids=1", map[string]string{
+		"action": "attach",
+		"id":     "3",
+	})
+	sess.MakeRequest(t, req, http.StatusNotFound)
+
+	// a label owned by the repo still attaches normally
+	req = NewRequestWithValues(t, "POST", "/user2/repo1/issues/labels?issue_ids=1", map[string]string{
+		"action": "attach",
+		"id":     "2",
+	})
+	sess.MakeRequest(t, req, http.StatusOK)
+}