Add session export to PCAP

pcap.py: build_pcap(packets) — build PCAP bytes in memory (same format
as PcapWriter)
database.py: get_messages_raw() — raw elapsed/type/data without decode
server.py: GET /api/sessions/<id>/pcap generates a GSMTAP-encapsulated
PCAP (ATR/TPDU) with real timestamps and downloads it as
<session-name>.pcap (ASCII filename fallback + RFC 5987 UTF-8 for
spaces/unicode via _content_disposition)

tests: build_pcap + _content_disposition (67 total)

simtrace2_pysniff/server/database.py

@@ -160,6 +160,12 @@ class Database:
         initial_prev = self._last_tpdu_context(session_id, after_id + 1)
         return self._decode_rows(rows, initial_prev=initial_prev)
 
+    def get_messages_raw(self, session_id):
+        rows = self._conn.execute(
+            'SELECT elapsed, type, data FROM messages WHERE session_id=? ORDER BY elapsed, id',
+            (session_id,)).fetchall()
+        return [{'elapsed': r[0], 'type': r[1], 'data': r[2]} for r in rows]
+
     def count_messages(self, session_id):
         row = self._conn.execute(
             'SELECT COUNT(*) FROM messages WHERE session_id=?',