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/arch/metadata_test.go

@@ -10,6 +10,9 @@ import (
 	"io"
 	"testing"
 
+	"gitea.dev/modules/test"
+	"gitea.dev/modules/util"
+
 	"github.com/klauspost/compress/zstd"
 	"github.com/stretchr/testify/assert"
 	"github.com/ulikunitz/xz"
@@ -167,3 +170,25 @@ func TestParsePackageInfo(t *testing.T) {
 		assert.ElementsMatch(t, []string{"usr/bin/paket1"}, p.FileMetadata.Backup)
 	})
 }
+
+// TestParsePackageTooManyFiles ensures the accumulated file list is bounded to prevent
+// metadata amplification from a package with a huge number of (tiny) file entries.
+func TestParsePackageTooManyFiles(t *testing.T) {
+	defer test.MockVariableValue(&maxFileEntries, 3)()
+	buf := test.WriteTarCompression(func(w io.Writer) io.WriteCloser { return gzip.NewWriter(w) }, map[string]string{
+		"file1":    "content1",
+		".PKGINFO": string(createPKGINFOContent(packageName, packageVersion)),
+	})
+	_, err := ParsePackage(buf)
+	assert.NoError(t, err)
+
+	buf = test.WriteTarCompression(func(w io.Writer) io.WriteCloser { return gzip.NewWriter(w) }, map[string]string{
+		"file1":    "content1",
+		"file2":    "content2",
+		"file3":    "content3",
+		"file4":    "content4",
+		".PKGINFO": string(createPKGINFOContent(packageName, packageVersion)),
+	})
+	_, err = ParsePackage(buf)
+	assert.ErrorIs(t, err, util.ErrInvalidArgument)
+}