fix: various security fixes (#38406) (#38426)

Backport #38406 by @bircni

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: bircni <bircni@icloud.com>
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>

models/asymkey/ssh_key_test.go

@@ -473,10 +473,20 @@ func runErr(t *testing.T, stdin []byte, args ...string) {
 	}
 }
 
-func Test_PublicKeysAreExternallyManaged(t *testing.T) {
+func TestPublicKeysAreExternallyManaged(t *testing.T) {
 	key1 := unittest.AssertExistsAndLoadBean(t, &PublicKey{ID: 1})
 	externals, err := PublicKeysAreExternallyManaged(t.Context(), []*PublicKey{key1})
 	assert.NoError(t, err)
 	assert.Len(t, externals, 1)
 	assert.False(t, externals[0])
 }
+
+// TestCheckPublicKeyStringOversized tests if oversized SSH2 public key strings are rejected before triggering costly operations.
+func TestCheckPublicKeyStringOversized(t *testing.T) {
+	_, err := parseKeyString(strings.Repeat("a", maxKeyContentBytes+1))
+	assert.ErrorContains(t, err, "SSH public key content is too long")
+
+	content := "---- BEGIN SSH2 PUBLIC KEY ----\n" + strings.Repeat("a", maxKeyContentBase64Bytes+1) + "\n--- END SSH2 PUBLIC KEY ----"
+	_, err = parseKeyString(content)
+	assert.ErrorContains(t, err, "SSH public key base64 is too long")
+}