Add PCAP/PCAPNG import via POST /api/sessions/import

- pcap.py: parse_pcap (micro/nano, both byte orders) and parse_pcapng
  (SHB byte-order, IDB linktype + if_tsresol, EPB/SPB) readers that
  unwrap Ethernet/RAW/IPv4/NULL/SLL/SLL2 to UDP and extract GSMTAP-SIM
  packets (APDU->tpdu, ATR->atr, PPS->pps), with a heuristic fallback
- database: insert_messages bulk-insert helper
- server: import endpoint reads raw body, auto-detects format, skips
  non-GSMTAP packets; 400 'No GSMTAP data found in trace' (no session)
  when empty, else creates a 'pcap' session named after the file

Tests: 102 (pcap round-trip, pcapng, no-GSMTAP, endpoint 200/400).

simtrace2_pysniff/server/database.py

@@ -134,6 +134,14 @@ class Database:
         self._conn.commit()
         return cur.lastrowid
 
+    def insert_messages(self, session_id, rows):
+        """Bulk-insert messages; *rows* is an iterable of
+        (elapsed, msg_type, data, flags) tuples.  Returns the last rowid."""
+        self._conn.executemany(
+            'INSERT INTO messages (session_id, elapsed, type, data, flags) VALUES (?,?,?,?,?)',
+            [(session_id, elapsed, msg_type, data, flags) for elapsed, msg_type, data, flags in rows])
+        self._conn.commit()
+
     def get_message(self, msg_id):
         row = self._conn.execute(
             'SELECT id, session_id, elapsed, type, data, flags FROM messages WHERE id=?',