v0.112.0 — E-2: GET /disks flags the backup-target drive

Additive backup_target field, true for the drive backing the PRIMARY tier.

The controller cannot work this out itself: settings.StoragePath.BackupTarget is
customer INTENT, and on the two boxes migrated by hand in E-1 that intent was
never recorded -- intent is empty while the drive really IS the target. Without
this flag the absent-target alarm could not name the drive on exactly the boxes
that currently have one.

omitempty + false on an older agent, so an old controller degrades to the generic
disconnect alarm rather than a wrong one.

Test asserts the target IS flagged AND the non-target is NOT, as a pair -- a
blanket true would satisfy a naive one-sided check.

internal/localapi/backup_target_guard_test.go

@@ -2,6 +2,7 @@ package localapi
 
 import (
 	"context"
+	"encoding/json"
 	"io"
 	"log/slog"
 	"net/http"
@@ -106,3 +107,41 @@ func TestEjectStillAllowedOnANonTargetDrive(t *testing.T) {
 			"it must block exactly the target, not harden the whole eject path; got: %s", rr.Body.String())
 	}
 }
+
+// E-2 — GET /disks must FLAG the backup-target drive, because the controller cannot work it out.
+//
+// The controller's own StoragePath.BackupTarget is customer INTENT, and on a box migrated by hand
+// (E-1, both demo boxes) nobody ever assigned it — intent is empty while the drive really is the
+// target. Without this flag the absent-target alarm could not name the drive on exactly the boxes
+// that have one, which is the only place it currently matters.
+func TestDisksFlagsTheBackupTargetDrive(t *testing.T) {
+	h := backupTargetServer(t)
+	rr := do(t, h, http.MethodGet, "/disks", "A", "")
+	if rr.Code != http.StatusOK {
+		t.Fatalf("GET /disks = %d, body %s", rr.Code, rr.Body.String())
+	}
+	body := rr.Body.String()
+	// The target must be flagged and the non-target must not be — asserted as a pair, since a
+	// blanket true would satisfy a naive "is it flagged?" check.
+	var got struct {
+		Data struct {
+			Disks []struct {
+				Name         string `json:"name"`
+				BackupTarget bool   `json:"backup_target"`
+			} `json:"disks"`
+		} `json:"data"`
+	}
+	if err := json.Unmarshal([]byte(body), &got); err != nil {
+		t.Fatalf("decode: %v (body %s)", err, body)
+	}
+	seen := map[string]bool{}
+	for _, d := range got.Data.Disks {
+		seen[d.Name] = d.BackupTarget
+	}
+	if !seen["felhom-backup"] {
+		t.Errorf("felhom-backup is the primary tier's storage but backup_target is false; body: %s", body)
+	}
+	if seen["spare-drive"] {
+		t.Errorf("spare-drive is NOT the target but was flagged — a blanket true is not a signal; body: %s", body)
+	}
+}