Fix two preflight defects a live production rehearsal found

Ran the rehearsal read-only against a live production instance. It
completed, and two preflight checks were wrong in ways a test instance
could not have shown.

1. Store-backend detection missed the config entirely. A config generated
   by `stalwart --init` declares `type = "rocksdb"` inside a
   `[store.rocksdb]` section, which is what every test fixture here used.
   The production config has no section headers at all and declares
   `store.rocksdb.type = "rocksdb"` flat. Detection only matched a bare
   `type` key, so it reported "no known store backend type found in config"
   and left topology.store_backend empty.

   That mattered more than a warning suggests: backup.Run treated an
   unrecognized backend as a *skip*, so a real run would have continued
   with no filesystem or database backup at all - the single artifact that
   phase exists to produce, quietly absent. Flat dotted keys are now
   detected, and an unrecognized backend is a hard failure rather than a
   skip.

2. The cluster warning didn't say where it matched. It fires on any
   occurrence of "cluster" anywhere in the config, which is the correct
   bias - a missed cluster corrupts a shared store - but on the production
   config the only match was inside the value of an unrelated setting,
   leaving a whole config to search to establish that. It now names the
   location and distinguishes a match in the setting name from one in its
   value.

Both verified against the real config: store-backend now reports
"rocksdb (store.rocksdb)", and the cluster warning names the setting,
making a false positive dismissible at a glance.

No production data is in this commit: the fixtures use example.com and the
flat-key shape only. Coverage numbers from that run matched the earlier
scrubbed-corpus measurement exactly.

internal/preflight/cluster_test.go

@@ -6,6 +6,7 @@ package preflight
 import (
 	"os"
 	"path/filepath"
+	"strings"
 	"testing"
 )
 
@@ -35,3 +36,28 @@ func TestLooksClustered(t *testing.T) {
 		})
 	}
 }
+
+// Against a real instance the only match was inside the value of an
+// unrelated setting. A bare "config mentions clustering" left a whole
+// config to search; naming the location makes it dismissible at a glance.
+func TestClusterMentionsSaysWhereItMatched(t *testing.T) {
+	dir := t.TempDir()
+	path := filepath.Join(dir, "config.toml")
+	body := "server.hostname = \"mail.example.com\"\nconfig.local-keys.08 = \"cluster.*\"\n"
+	if err := os.WriteFile(path, []byte(body), 0o640); err != nil {
+		t.Fatal(err)
+	}
+	mentions, err := ClusterMentions(path)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if len(mentions) != 1 {
+		t.Fatalf("mentions = %v, want one", mentions)
+	}
+	if !strings.Contains(mentions[0], "config.local-keys.08") {
+		t.Errorf("mention = %q, should name the setting", mentions[0])
+	}
+	if !strings.Contains(mentions[0], "in its value") {
+		t.Errorf("mention = %q, should distinguish a value match from a real cluster setting", mentions[0])
+	}
+}