fix(retention): a tiny or brand-new retention value can't wipe a room (#298)

m.room.retention can be set to anything by anyone with state power. A
fat-fingered `max_lifetime: 1000` blanked the timeline for every Lotus
user and, for members with "Enforce Message Retention" on, the sweeper
redacted all their messages within a minute. The RETENTION_MIN_MS floor
existed but was never applied.

- effectiveRetentionMs(): anything under 10 min counts as off; used by
  getRoomRetentionMs (sweeper) and the timeline's hiding.
- The sweeper never deletes under a policy less than 24 h old, so a
  mistake or a malicious change can be reverted before anything is lost.
  (Local hiding of expired messages still applies at once.)

Verified headless (the issue's repro, enforcement on): after
max_lifetime 1000, all 4 messages stay visible and none are redacted
after 45 s (before: timeline blank, all 4 redacted). Unit tests for the
floor and the grace window.

Fixes #298

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017PCb59uY3UKqDqt7B6hzd5

src/app/utils/retention.ts

@@ -20,13 +20,38 @@ export const RETENTION_PRESETS: RetentionPreset[] = [
   { label: '1 Month', ms: 30 * DAY_MS },
 ];
 
+/**
+ * [Gitea #298] A usable retention window, or `undefined`. Anything under
+ * RETENTION_MIN_MS is treated as off: a fat-fingered `max_lifetime: 1000` (set
+ * from any client by anyone with state power) used to blank every timeline and
+ * get opted-in users' messages redacted within a minute.
+ */
+export const effectiveRetentionMs = (maxLifetime: unknown): number | undefined =>
+  typeof maxLifetime === 'number' && Number.isFinite(maxLifetime) && maxLifetime >= RETENTION_MIN_MS
+    ? maxLifetime
+    : undefined;
+
 /** The room's active retention window in ms, or `undefined` when unset/disabled. */
 export const getRoomRetentionMs = (room: Room): number | undefined => {
   const event = room.currentState.getStateEvents(StateEvent.RoomRetention, '');
-  const ms = event?.getContent<RetentionContent>()?.max_lifetime;
-  return typeof ms === 'number' && ms > 0 ? ms : undefined;
+  return effectiveRetentionMs(event?.getContent<RetentionContent>()?.max_lifetime);
 };
 
+/**
+ * [Gitea #298] Deletion grace: the auto-delete sweeper ignores a retention
+ * policy for this long after it was set, so a mistaken or malicious change can
+ * be reverted before anything is permanently redacted. (Hiding expired
+ * messages locally is not destructive and applies at once.)
+ */
+export const RETENTION_DELETE_GRACE_MS = DAY_MS;
+
+export const retentionDeletesAllowed = (policySetAtMs: number, nowMs: number): boolean =>
+  nowMs - policySetAtMs >= RETENTION_DELETE_GRACE_MS;
+
+/** When the room's current retention policy was set (its state event's ts). */
+export const getRoomRetentionSetAt = (room: Room): number | undefined =>
+  room.currentState.getStateEvents(StateEvent.RoomRetention, '')?.getTs();
+
 /** True when an event at `tsMs` has passed the `maxLifetimeMs` retention window. */
 export const isExpired = (tsMs: number, maxLifetimeMs: number, nowMs: number): boolean =>
   maxLifetimeMs > 0 && nowMs - tsMs > maxLifetimeMs;