v0.185.1 — E-2: the offer endpoints were mounted where nothing routed to them

Registered as /api/backup-target inside ServeStorageAPI, which main.go mounts ONLY
at /api/storage/. Live result: endpoint not found, while every unit test passed --
the tests called the handlers directly and never travelled the mount. Caught by
the first live call, which is why the live call is part of the procedure.

Moved to /api/storage/backup-target[/assign]. A new test asserts the dispatcher
source contains both paths, so a handler nothing routes to fails the suite --
the seam-wiring rule applied to a route rather than a button.

controller/internal/web/backup_target_offer_test.go

@@ -1,6 +1,7 @@
 package web
 
 import (
+	"os"
 	"strings"
 	"testing"
 )
@@ -59,3 +60,39 @@ func TestUnknownStateRendersNothing(t *testing.T) {
 			"of degradation", msg)
 	}
 }
+
+// REACHABILITY — the repo's seam-wiring rule, and it already bit once here.
+//
+// These handlers were first registered as /api/backup-target inside ServeStorageAPI's switch, which
+// main.go mounts ONLY at "/api/storage/". The endpoints returned "endpoint not found" on a live box
+// while every unit test passed, because the tests called the handlers directly and never travelled
+// the mount. A feature is not shipped until its entry point is reachable.
+func TestBackupTargetRoutesLiveUnderTheStorageAPIMount(t *testing.T) {
+	// main.go: mux.Handle("/api/storage/", ... ServeStorageAPI). Anything the switch answers MUST
+	// therefore begin with that prefix or it is dead on arrival.
+	for _, p := range []string{
+		"/api/storage/backup-target",
+		"/api/storage/backup-target/assign",
+	} {
+		if !strings.HasPrefix(p, "/api/storage/") {
+			t.Fatalf("%s is outside the /api/storage/ mount and would 404 in production", p)
+		}
+	}
+	src := storageAPISource(t)
+	for _, want := range []string{`"/api/storage/backup-target"`, `"/api/storage/backup-target/assign"`} {
+		if !strings.Contains(src, want) {
+			t.Errorf("ServeStorageAPI does not dispatch %s — the handler exists but nothing routes to it", want)
+		}
+	}
+}
+
+// storageAPISource reads the dispatcher's own source, so the assertion is about what the router
+// actually contains rather than about a constant the test itself defines.
+func storageAPISource(t *testing.T) string {
+	t.Helper()
+	b, err := os.ReadFile("storage_handlers.go")
+	if err != nil {
+		t.Fatalf("read storage_handlers.go: %v", err)
+	}
+	return string(b)
+}