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)

tests/test_pcap.py

@@ -0,0 +1,60 @@
+"""Tests for PCAP generation and Content-Disposition filename."""
+
+import struct
+import unittest
+
+from simtrace2_pysniff.pcap import build_pcap, PCAP_MAGIC, LINKTYPE_GSMTAP
+from simtrace2_pysniff.server.server import _content_disposition
+
+
+class TestBuildPcap(unittest.TestCase):
+    def _gsmtap_hdr(self, sub_type):
+        import struct
+        return struct.pack('!BBBBHBBIBBBB', 2, 4, 7, 0, 0, 0, 0, 0, sub_type, 0, 0, 0)
+
+    def test_empty(self):
+        data = build_pcap([])
+        self.assertEqual(len(data), 24)  # just the global header
+        magic, vmaj, vmin, _tz, _sig, snaplen, network = struct.unpack('<IHHiIII', data[:24])
+        self.assertEqual(magic, PCAP_MAGIC)
+        self.assertEqual((vmaj, vmin), (2, 4))
+        self.assertEqual(snaplen, 65535)
+        self.assertEqual(network, LINKTYPE_GSMTAP)
+
+    def test_one_packet(self):
+        pkt = build_pcap([(self._gsmtap_hdr(2), b'\xa0\xa4\x00\x00\x02\x3f\x00', 1234567890.5)])
+        self.assertEqual(len(pkt), 24 + 16 + (16 + 7))  # global + pkt hdr + gsmtap hdr + data
+        ts_sec, ts_usec, incl_len, orig_len = struct.unpack('<IIII', data_bytes := pkt[24:40])
+        self.assertEqual(ts_sec, 1234567890)
+        self.assertEqual(ts_usec, 500000)
+        self.assertEqual(incl_len, 23)
+        self.assertEqual(orig_len, 23)
+
+    def test_multiple_packets(self):
+        packets = [
+            (self._gsmtap_hdr(1), b'\x3b\x00', 1.0),
+            (self._gsmtap_hdr(2), b'\x80\xf2\x00\x00\x00', 2.0),
+        ]
+        data = build_pcap(packets)
+        # global header + 2 packets (each: 16 pkt hdr + 16 gsmtap hdr + data)
+        self.assertEqual(len(data), 24 + (16 + 16 + 2) + (16 + 16 + 5))
+
+
+class TestContentDisposition(unittest.TestCase):
+    def test_ascii(self):
+        h = _content_disposition('My Capture.pcap')
+        self.assertIn('filename="My Capture.pcap"', h)
+
+    def test_unicode(self):
+        h = _content_disposition('Моя сессия.pcap')
+        # ASCII fallback uses '?' replacements; UTF-8 filename* carries the real name
+        self.assertIn("filename*=UTF-8''", h)
+        self.assertIn('%D0%9C', h)  # 'М' = %D0%9C
+
+    def test_special_chars(self):
+        h = _content_disposition('a"b.pcap')
+        self.assertIn('filename="a_b.pcap"', h)
+
+
+if __name__ == '__main__':
+    unittest.main()