Add KeepOrg sync server: accounts, nested groups, versioned KDBX sync

Why: collaborative KeePass use needs one arbiter of "who wrote last" that
never sees a master password, plus access control that follows an org chart.

What: stdlib-only Go server. Metadata is a single JSON document with one
writer and atomic commit; KDBX payloads are versioned blobs on disk. Access
comes from a group tree where a grant on a group reaches every member below
it. Login is password+TOTP, invite links, or OIDC through a browser-approved
device flow. Pushes carry If-Match and are arbitrated by a per-database merge
strategy (server-wins by default). Updates are pushed to clients over SSE.

Proved: go test ./internal/... covers RFC 6238 vectors, ACL inheritance and
isolation, store rollback, blob pruning, every push verdict, and an
end-to-end HTTP pass over login, conflict, force, read-only grants, invite
redemption, TOTP enrolment, device login and the event stream.

internal/secret/secret_test.go

@@ -0,0 +1,31 @@
+package secret
+
+import "testing"
+
+func TestHashVerifyRoundTrip(t *testing.T) {
+	h, err := Hash("correct horse")
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !Verify(h, "correct horse") {
+		t.Fatal("correct password rejected")
+	}
+	if Verify(h, "wrong horse") {
+		t.Fatal("wrong password accepted")
+	}
+}
+
+func TestVerifyRejectsJunkVerifier(t *testing.T) {
+	for _, v := range []string{"", "x", "pbkdf2-sha256$1$a$b", "md5$1$a$b"} {
+		if Verify(v, "anything") {
+			t.Fatalf("verifier %q accepted", v)
+		}
+	}
+}
+
+func TestUserCodeShape(t *testing.T) {
+	c := UserCode()
+	if len(c) != 9 || c[4] != '-' {
+		t.Fatalf("user code %q", c)
+	}
+}