v0.317.0: a second copy counts only on a proven different disk; the no-drive install speaks in words (felhom.eu architecture/16, build A)

Tier-2 and the one-copy notice ask the host agent (POST /disks/same-disk, agent 0.157.0): different +
proven -> made and counted; same disk -> not made (D1); unknown (VM, hidden USB serial) -> made, not
counted, the row says why (D2); a NAS never counts and no longer hides the notice (R-1075). An older
agent keeps the st_dev rule, logged once. D4: a data-folder or media app on a box with no drive is
refused in words with a link to the drives page (R-1068's raw-error half). MinAgent 0.157.0.

Claude-Session: https://claude.ai/code/session_012qRErfCoiTkvDK9N5XHbzb

controller/internal/api/d4_needs_drive_test.go

@@ -0,0 +1,17 @@
+package api
+
+import (
+	"net/http"
+	"testing"
+
+	"gitea.dooplex.hu/admin/felhom-controller/internal/stacks"
+	"gitea.dooplex.hu/admin/felhom-controller/internal/util"
+)
+
+// D4 (`16` §5): the no-drive refusal is a 400 the household can act on — not a 500.
+func TestD4_NeedsDriveIs400(t *testing.T) {
+	err := util.MsgErrorf(stacks.ErrNeedsDrive, "err.stacks.needs_drive")
+	if got := deployStatusFor(err); got != http.StatusBadRequest {
+		t.Fatalf("ErrNeedsDrive → %d, want 400", got)
+	}
+}