Fix imported PCAP session duration for relative timestamps

SIGROK-derived PCAPs have relative timestamps (0, 1.37, 15.02) rather
than epoch time. The _REAL_TS_MIN check failed, so close_session() set
ended to wall-clock (same as started → duration 0s). Now close_session
accepts an optional duration_secs param; the import handler passes
last_ts - first_ts so the session gets the correct capture duration.

simtrace2_pysniff/server/server.py

@@ -346,7 +346,8 @@ class RequestHandler(BaseHTTPRequestHandler):
         if first_ts is not None and first_ts > _REAL_TS_MIN:
             self.db.set_session_times_from_ts(session_id, first_ts, last_ts)
         else:
-            self.db.close_session(session_id)
+            duration = (last_ts - first_ts) if last_ts is not None and first_ts is not None else 0
+            self.db.close_session(session_id, duration)
 
         _log(f'Session imported: session={session_id} '
              f'name={name or "(untitled)"} messages={len(packets)} '