feat(topology): persist comparison runs + history

Save each comparison and let users reload past ones.
- migration 0008: topology_runs (workspace-scoped; full comparison as JSONB).
- cm-db repo::topology_runs (insert / list_recent / get) + regenerated .sqlx.
- cm-api: compare persists best-effort (never loses the LLM result on a DB
  hiccup); GET /api/topology-runs (recent) + GET /api/topology-runs/{id}.
  Integration test asserts persist → list → get.
- frontend: "Recent comparisons" list on the Compare tab; click to reload a
  saved run. e2e p8 green (39 suite); offline build + clippy clean.

Server self-migrates at boot (cm_db::MIGRATOR), so 0008 applies on deploy.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

frontend/src/components/topology/TopologyCompare.tsx

@@ -1,9 +1,15 @@
 "use client";
 
-import { useState } from "react";
+import { useEffect, useState } from "react";
 
 import type { CatalogEntry, TopologyGraph } from "@/lib/api/topology";
 
+interface RunSummary {
+  id: string;
+  task: string;
+  created_at: string;
+}
+
 interface CompareResult {
   kind: string;
   quality: number;
@@ -28,6 +34,33 @@ export function TopologyCompare({ catalog }: { catalog: CatalogEntry[] }) {
   const [cmp, setCmp] = useState<Comparison | null>(null);
   const [busy, setBusy] = useState(false);
   const [error, setError] = useState<string | null>(null);
+  const [runs, setRuns] = useState<RunSummary[]>([]);
+
+  async function loadRuns() {
+    try {
+      const res = await fetch("/api/topology-runs");
+      if (res.ok) setRuns((await res.json()) as RunSummary[]);
+    } catch {
+      /* recent list is best-effort */
+    }
+  }
+
+  useEffect(() => {
+    void loadRuns();
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, []);
+
+  async function loadRun(id: string) {
+    try {
+      const res = await fetch(`/api/topology-runs/${id}`);
+      if (res.ok) {
+        const detail = (await res.json()) as { comparison: Comparison };
+        setCmp(detail.comparison);
+      }
+    } catch {
+      /* ignore */
+    }
+  }
 
   function toggle(kind: string) {
     setSelected((s) => {
@@ -63,6 +96,7 @@ export function TopologyCompare({ catalog }: { catalog: CatalogEntry[] }) {
       });
       if (!res.ok) throw new Error(`Compare failed (${res.status})`);
       setCmp((await res.json()) as Comparison);
+      void loadRuns();
     } catch (e) {
       setError(e instanceof Error ? e.message : "Compare failed");
     } finally {
@@ -120,6 +154,22 @@ export function TopologyCompare({ catalog }: { catalog: CatalogEntry[] }) {
       {error && <p className="text-sm text-coral">{error}</p>}
 
       {cmp && <Results cmp={cmp} />}
+
+      {runs.length > 0 && (
+        <div className="flex flex-col gap-1 border-t border-border pt-4">
+          <p className="text-xs font-medium text-muted-foreground">Recent comparisons</p>
+          {runs.map((r) => (
+            <button
+              key={r.id}
+              type="button"
+              onClick={() => loadRun(r.id)}
+              className="rounded-lg px-3 py-1.5 text-left text-xs text-muted-foreground transition-colors hover:bg-surface-warm/50"
+            >
+              {r.task}
+            </button>
+          ))}
+        </div>
+      )}
     </div>
   );
 }