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/packages/filelist.go

@@ -0,0 +1,37 @@
+// Copyright 2026 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package packages
+
+import "gitea.dev/modules/util"
+
+// BoundedFileList accumulates file names from a package archive while enforcing caps on the number of
+// entries and their total name length, returning an error once either cap would be exceeded.
+type BoundedFileList struct {
+	files     []string
+	nameBytes int
+	maxFiles  int
+	maxBytes  int
+}
+
+// NewBoundedFileList creates a BoundedFileList with the given caps; a non-positive cap falls back to the
+// corresponding default.
+func NewBoundedFileList(maxFiles, maxNameBytes int) *BoundedFileList {
+	return &BoundedFileList{maxFiles: maxFiles, maxBytes: maxNameBytes}
+}
+
+// Add appends name, returning util.ErrInvalidArgument once the entry count or accumulated byte length
+// would exceed the configured cap.
+func (b *BoundedFileList) Add(name string) error {
+	if len(b.files) >= b.maxFiles || b.nameBytes+len(name) > b.maxBytes {
+		return util.NewInvalidArgumentErrorf("package contains too many file entries")
+	}
+	b.nameBytes += len(name)
+	b.files = append(b.files, name)
+	return nil
+}
+
+// Files returns the accumulated file names.
+func (b *BoundedFileList) Files() []string {
+	return b.files
+}