channels: drop stale membership caches on join/leave

After channels.leaveChannel, a client that polls channels.getFullChannel kept
receiving a projection that still showed it as an active member (left=false)
until the per-(viewer,channel) RPC projection cache and the store-level member
cache lapsed on their own or the async read-model NOTIFY landed. The client
therefore kept an open compose box while every send was already rejected with
CHANNEL_PRIVATE - most visible on public forum supergroups, where getFullChannel
keeps succeeding via the preview path instead of tearing the chat down.

Every other membership-mutating path already busts these caches synchronously;
join/leave/invite/request-approval did not. Add:

- store: invalidateChannelMembershipCaches (row + member + dialog caches),
  called post-commit from JoinChannel, LeaveChannel, ImportInvite,
  InviteToChannel.
- rpc: invalidateChannelMembershipProjection (channelFullProjectionCache pair),
  called from the join/leave/invite/hide-requests handlers for every user whose
  membership changed.

internal/store/postgres/channel_store.go

@@ -118,6 +118,33 @@ func (s *ChannelStore) boostCacheActive(db sqlcgen.DBTX) bool {
 	return s.boostCache != nil && db == s.db
 }
 
+// invalidateChannelMembershipCaches drops the in-process reads that a membership
+// change (join, leave, invite, kick) makes stale for the given users. The
+// ReadModelChangeListener also clears these off the async NOTIFY, but callers
+// must not depend on that round-trip: a client that polls channels.getFullChannel
+// right after channels.leaveChannel would otherwise keep seeing itself as an
+// active member (and keep an open compose box) until the notify lands. Call it
+// post-commit.
+func (s *ChannelStore) invalidateChannelMembershipCaches(channelID int64, userIDs ...int64) {
+	if channelID == 0 {
+		return
+	}
+	if s.rowCache != nil {
+		s.rowCache.delete(channelID)
+	}
+	for _, userID := range userIDs {
+		if userID == 0 {
+			continue
+		}
+		if s.memberCache != nil {
+			s.memberCache.delete(channelID, userID)
+		}
+		if s.dialogCache != nil {
+			s.dialogCache.delete(userID, channelID)
+		}
+	}
+}
+
 // NewChannelStore 基于 pgx 连接池（或事务）创建 ChannelStore。
 func NewChannelStore(db sqlcgen.DBTX, opts ...ChannelStoreOption) *ChannelStore {
 	s := &ChannelStore{db: db}