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>

models/auth/access_token_scope_test.go

@@ -89,3 +89,26 @@ func TestAccessTokenScope_HasScope(t *testing.T) {
 		})
 	}
 }
+
+func TestAccessTokenScope_EnforcePublicOnlyFrom(t *testing.T) {
+	tests := []struct {
+		in     AccessTokenScope
+		parent AccessTokenScope
+		out    AccessTokenScope
+	}{
+		// public-only parent forces the restriction onto the minted scope
+		{"write:user", "write:user,public-only", "public-only,write:user"},
+		// already public-only stays public-only
+		{"public-only,read:user", "public-only", "public-only,read:user"},
+		// non-public-only parent leaves the scope untouched
+		{"write:user", "write:user", "write:user"},
+		{"all", "all", "all"},
+	}
+	for _, test := range tests {
+		t.Run(string(test.parent)+"->"+string(test.in), func(t *testing.T) {
+			got, err := test.in.EnforcePublicOnlyFrom(test.parent)
+			assert.NoError(t, err)
+			assert.Equal(t, test.out, got)
+		})
+	}
+}