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>

docs/mongodb.md

@@ -121,6 +121,28 @@ Key fields:
 Primary use: accepted and dismissed outcomes drive exact recall, suppression,
 and learned graph paths.
 
+### `suppressions`
+
+Durable negative-feedback proof: one record per *suppressed repeat* — a
+previously-dismissed collision signature recurred and PodMan stayed quiet.
+Written at repeat time by `backend/src/agent/podman.ts` (once per recurrence,
+re-armed on resolution), materialized as `suppressed` activity by
+`backend/src/graph/live.ts`.
+
+Key fields:
+
+- `id`
+- `podId`
+- `collisionId`
+- `file`
+- `engineers`
+- `priorInterventionId` — the dismissed intervention this repeat matched
+- `priorDismissedAt`
+- `suppressedAt` — the repeat time (drives recency in the activity stream)
+
+Index `{ podId: 1, suppressedAt: -1 }`. Counted in `/api/memory/stats`.
+**Preserve in any DB cleanup** — this is visible learning evidence, not noise.
+
 ### `team_model`
 
 Durable per-pod summary memory.