fix(compaction): use DB count for threshold check after server restart

MaybeCompact relied on RAM count which resets to 0 on restart (LoadFromDB
is a no-op). Messages accumulated in DB but never triggered compaction.
Now falls back to CountByKey DB query when RAM count is below threshold.

internal/channels/history_compaction.go

@@ -24,6 +24,7 @@ type CompactionConfig struct {
 
 // MaybeCompact checks if compaction is needed for a history key and triggers it in background.
 // Called from Record() after appending. Thread-safe via sync.Map compaction guard.
+// Uses DB count (not RAM count) to correctly detect threshold after server restarts.
 func (ph *PendingHistory) MaybeCompact(historyKey string, currentCount int, cfg *CompactionConfig) {
 	if ph.store == nil || cfg == nil || cfg.Provider == nil {
 		return
@@ -32,9 +33,18 @@ func (ph *PendingHistory) MaybeCompact(historyKey string, currentCount int, cfg
 	if threshold <= 0 {
 		threshold = DefaultGroupHistoryLimit
 	}
+
+	// RAM count may be stale after restart (LoadFromDB doesn't warm full history).
+	// If RAM says below threshold, ask DB for the real count.
 	if currentCount <= threshold {
-		return
+		ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+		defer cancel()
+		dbCount, err := ph.store.CountByKey(ctx, ph.channelName, historyKey)
+		if err != nil || dbCount <= threshold {
+			return
+		}
 	}
+
 	// Guard: only one compaction per key at a time
 	if _, loaded := ph.compacting.LoadOrStore(historyKey, true); loaded {
 		return

internal/store/pending_message_store.go

@@ -55,6 +55,9 @@ type PendingMessageStore interface {
 	// CountAll returns the total number of pending messages across all groups.
 	CountAll(ctx context.Context) (int64, error)
 
+	// CountByKey returns the number of pending messages for a specific channel+historyKey.
+	CountByKey(ctx context.Context, channelName, historyKey string) (int, error)
+
 	// ResolveGroupTitles looks up chat_title from session metadata for each group.
 	// Returns a map of "channel_name:history_key" → title. Used only by the UI layer.
 	ResolveGroupTitles(ctx context.Context, groups []PendingMessageGroup) (map[string]string, error)

internal/store/pg/pending_message_store.go

@@ -191,6 +191,15 @@ func (s *PGPendingMessageStore) CountAll(ctx context.Context) (int64, error) {
 	return count, err
 }
 
+func (s *PGPendingMessageStore) CountByKey(ctx context.Context, channelName, historyKey string) (int, error) {
+	var count int
+	err := s.db.QueryRowContext(ctx,
+		`SELECT COUNT(*) FROM channel_pending_messages WHERE channel_name = $1 AND history_key = $2`,
+		channelName, historyKey,
+	).Scan(&count)
+	return count, err
+}
+
 func (s *PGPendingMessageStore) ResolveGroupTitles(ctx context.Context, groups []store.PendingMessageGroup) (map[string]string, error) {
 	if len(groups) == 0 {
 		return nil, nil