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/client.py

@@ -10,15 +10,20 @@ The client does not hold an open connection between calls.  This keeps the
 first implementation simple and matches Blastware's observed behaviour.
 Persistent connections can be added later without changing the public API.
 
-Example:
+Example (serial):
     from minimateplus import MiniMateClient
 
     with MiniMateClient("COM5") as device:
         info   = device.connect()          # POLL handshake + identity read
         events = device.get_events()       # download all events
-        print(info)
-        for ev in events:
-            print(ev)
+
+Example (TCP / modem):
+    from minimateplus import MiniMateClient
+    from minimateplus.transport import TcpTransport
+
+    transport = TcpTransport("203.0.113.5", port=12345)
+    with MiniMateClient(transport=transport) as device:
+        info = device.connect()
 """
 
 from __future__ import annotations
@@ -55,16 +60,17 @@ class MiniMateClient:
     High-level client for a single MiniMate Plus device.
 
     Args:
-        port:    Serial port name (e.g. "COM5", "/dev/ttyUSB0").
-        baud:    Baud rate (default 38400).
-        timeout: Per-request receive timeout in seconds (default 5.0).
-        transport: Optional pre-built transport (for testing / TCP future use).
+        port:      Serial port name (e.g. "COM5", "/dev/ttyUSB0").
+                   Not required when a pre-built transport is provided.
+        baud:      Baud rate (default 38400, ignored when transport is provided).
+        timeout:   Per-request receive timeout in seconds (default 15.0).
+        transport: Pre-built transport (SerialTransport or TcpTransport).
                    If None, a SerialTransport is constructed from port/baud.
     """
 
     def __init__(
         self,
-        port: str,
+        port: str = "",
         baud: int = 38_400,
         timeout: float = 15.0,
         transport: Optional[BaseTransport] = None,