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/debian/metadata_test.go

@@ -232,3 +232,15 @@ func TestValidateDistributionOrComponent(t *testing.T) {
 		assert.True(t, IsValidDistributionOrComponent(name), "good=%q", name)
 	}
 }
+
+// TestParseControlFileMultilineDescription verifies a multi-line Description is assembled in order
+// (the parser accumulates it in a strings.Builder); it guards the assembled value, not its timing.
+func TestParseControlFileMultilineDescription(t *testing.T) {
+	var buf bytes.Buffer
+	buf.WriteString("Package: testpkg\nVersion: 1.0\nArchitecture: amd64\nDescription: short summary\n more details\n even more\n")
+
+	p, err := ParseControlFile(&buf)
+	assert.NoError(t, err)
+	assert.NotNil(t, p)
+	assert.Equal(t, "short summary more details even more", p.Metadata.Description)
+}