feat(heartbeat): support topic suffix in delivery targets

- Use array_to_string([5:]) in PG to capture full chatId with topic:N suffix
- Add extractSessionKeyTail() for SQLite forum group support
- Join on base chatId for contact display name resolution

internal/store/pg/heartbeat.go

@@ -276,12 +276,14 @@ func (s *PGHeartbeatStore) ListLogs(ctx context.Context, agentID uuid.UUID, limi
 
 // ListDeliveryTargets returns distinct (channel, chatID, title, kind) pairs from sessions for an agent.
 // Uses idx_sessions_agent (btree on agent_id) for fast lookup.
-// Session key format: agent:{key}:{channel}:{kind}:{chatId}
+// Session key format: agent:{key}:{channel}:{kind}:{chatId}[:topic:N]
+// Extracts full chatId including topic/thread suffix via array_to_string([5:]).
+// JOIN on base chatId (split_part segment 5) for contact display name resolution.
 func (s *PGHeartbeatStore) ListDeliveryTargets(ctx context.Context, agentID uuid.UUID) ([]store.DeliveryTarget, error) {
 	rows, err := s.db.QueryContext(ctx,
 		`SELECT DISTINCT ON (s.channel, chat_id)
 		    s.channel,
-		    split_part(s.session_key, ':', 5) AS chat_id,
+		    array_to_string((string_to_array(s.session_key::text, ':'))[5:], ':') AS chat_id,
 		    COALESCE(
 		        s.metadata->>'chat_title',
 		        cc.display_name,

internal/store/sqlitestore/heartbeat.go

@@ -319,8 +319,8 @@ func (s *SQLiteHeartbeatStore) ListDeliveryTargets(ctx context.Context, agentID
 			return nil, err
 		}
 
-		// Parse chatId: 5th colon-separated segment of session_key.
-		chatID := extractSessionKeyPart(sessionKey, 5)
+		// Parse chatId: everything after 4th colon (includes :topic:N suffix for forum groups).
+		chatID := extractSessionKeyTail(sessionKey, 5)
 		if chatID == "" {
 			continue
 		}
@@ -369,3 +369,14 @@ func extractSessionKeyPart(key string, n int) string {
 	}
 	return parts[n-1]
 }
+
+// extractSessionKeyTail returns everything from the n-th colon-delimited segment onwards.
+// For session key "agent:default:telegram:group:-100123:topic:42" with n=5,
+// returns "-100123:topic:42" (includes topic/thread suffix).
+func extractSessionKeyTail(key string, n int) string {
+	parts := strings.SplitN(key, ":", n)
+	if len(parts) < n {
+		return ""
+	}
+	return parts[n-1]
+}