Add TCP/modem transport (Sierra Wireless RV55/RX55 field units)

- minimateplus/transport.py: add TcpTransport — stdlib socket-based transport
  with same interface as SerialTransport. Overrides read_until_idle() with
  idle_gap=1.5s to absorb the modem's 1-second serial data forwarding buffer.
- minimateplus/client.py: make `port` param optional (default "") so
  MiniMateClient works cleanly when a pre-built transport is injected.
- minimateplus/__init__.py: export SerialTransport and TcpTransport.
- sfm/server.py: add `host` / `tcp_port` query params to all device endpoints.
  New _build_client() helper selects TCP or serial transport automatically.
  OSError (connection refused, timeout) now returns HTTP 502.
- docs/instantel_protocol_reference.md: add changelog entry and full §14
  (TCP/Modem Transport) documenting confirmed transparent passthrough, no ENQ
  on connect, modem forwarding delay, call-up vs ACH modes, and hardware note
  deprecating Raven X in favour of RV55/RX55.

Usage: GET /device/info?host=<modem_ip>&tcp_port=12345

minimateplus/__init__.py

@@ -2,18 +2,26 @@
 minimateplus — Instantel MiniMate Plus protocol library.
 
 Provides a clean Python API for communicating with MiniMate Plus seismographs
-over RS-232 serial (direct cable) or TCP (via RV50 cellular modem bridge).
+over RS-232 serial (direct cable) or TCP (modem / ACH Auto Call Home).
 
-Typical usage:
+Typical usage (serial):
     from minimateplus import MiniMateClient
 
     with MiniMateClient("COM5") as device:
         info   = device.connect()
         events = device.get_events()
+
+Typical usage (TCP / modem):
+    from minimateplus import MiniMateClient
+    from minimateplus.transport import TcpTransport
+
+    with MiniMateClient(transport=TcpTransport("203.0.113.5", 12345)) as device:
+        info = device.connect()
 """
 
 from .client import MiniMateClient
 from .models import DeviceInfo, Event
+from .transport import SerialTransport, TcpTransport
 
 __version__ = "0.1.0"
-__all__ = ["MiniMateClient", "DeviceInfo", "Event"]
+__all__ = ["MiniMateClient", "DeviceInfo", "Event", "SerialTransport", "TcpTransport"]