fix(graph): harden suppressed-repeat activity (Codex deep review) (#44)

Four demo-safety fixes on top of #43, none touching demo-pod data:

- De-dup at source: the suppression path returned before activeConflicts.add(),
  so the same unresolved dismissed collision wrote a SuppressionDoc every frame
  (~194 spam rows observed on prod). Now mark the conflict handled first, so it
  records ONCE per recurrence and re-arms via the onScreenFrame resolution sweep.
- Await the write: recordSuppression is the visible learning proof, so await it
  (like recordCollision/recordIntervention) instead of void ...catch().
- Display de-dup: collapse suppressed beats to one per file (most recent) with a
  stable per-file id, so any pre-fix duplicate rows never render as spam.
- Live-graph gate: suppression beats now satisfy the "has activity" check in
  materializePodGraph, so a clean pod with preserved suppressions (but no
  collision/file nodes) no longer falls back to the demo graph and hides the proof.
- Ops: /api/memory/stats counts `suppressions`; docs/mongodb.md documents the
  collection and flags it "preserve in DB cleanup" (visible learning evidence).

Verified end-to-end on a throwaway pod (not demo-pod): suppression-only pod
materializes; 2 dupe rows render as 1 beat. backend+frontend typecheck + eslint pass.

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/graph/live.ts

@@ -463,16 +463,23 @@ export async function materializePodGraph(podId: string): Promise<PodGraph | nul
   const suppressionDocs = await c.suppressions
     .find({ podId })
     .sort({ suppressedAt: -1 })
-    .limit(20)
+    .limit(50)
     .toArray();
+  // Collapse to one beat per file (keep the most recent — docs are sorted desc)
+  // so pre-fix duplicate rows never render as spam. The stable per-file id also
+  // dedupes through pushActivity's `seen` set.
+  const seenSuppressedFiles = new Set<string>();
   for (const s of suppressionDocs) {
     const sFile = normalizeFile(s.file);
     if (!isFilePath(sFile)) continue;
+    const fileKey = sFile.toLowerCase();
+    if (seenSuppressedFiles.has(fileKey)) continue;
+    seenSuppressedFiles.add(fileKey);
     const sEngs = (s.engineers ?? []).join(' + ') || 'teammates';
     pushActivity(
       activity,
       {
-        id: `suppressed:${s.id}`,
+        id: `suppressed:${fileKey}`,
         at: s.suppressedAt,
         kind: 'suppressed',
         title: `Suppressed — ${shortLabel(sFile)} repeat silenced`,
@@ -520,7 +527,11 @@ export async function materializePodGraph(podId: string): Promise<PodGraph | nul
 
   const nodes = [...b.nodes.values()];
   // No real activity beyond the bare roster -> let the caller fall back to demo.
-  const hasActivity = nodes.some((n) => n.kind !== 'engineer');
+  // Suppression beats are real negative-feedback proof even when they add no
+  // nodes/edges, so they satisfy the gate too — a clean pod with preserved
+  // suppressions must not fall back to the demo graph and hide the proof.
+  const hasSuppressed = activity.some((a) => a.kind === 'suppressed');
+  const hasActivity = hasSuppressed || nodes.some((n) => n.kind !== 'engineer');
   if (!hasActivity) return null;
 
   layout(nodes);