feat(api): FastAPI bridge for pont bascule via serial + Tailscale access

- Add FastAPI service exposing /health, /last, /send/esclave, /send/dsd, /send/custom
- Implement SerialBridge aligned with legacy Tkinter behavior (open delay 2s, post-write 0.5s, read in_waiting once)
- Enforce single in-flight serial request (non-blocking lock, returns 409 BUSY)
- Add environment-based serial configuration (.env + systemd EnvironmentFile)
- Document installation, systemd service, and Tailscale usage (direct IP and tailscale serve)

app/serial_bridge.py.old

@@ -0,0 +1,26 @@
+import threading
+from dataclasses import dataclass
+
+def hex2b(s: str) -> bytes:
+    return bytes(int(x, 16) for x in s.split())
+
+@dataclass
+class SerialConfig:
+    port: str
+    baudrate: int = 9600
+
+class SerialBridge:
+    def __init__(self, cfg: SerialConfig):
+        self.cfg = cfg
+        self._lock = threading.Lock()
+
+    def busy(self):
+        return self._lock.locked()
+
+    def send_and_read_once(self, payload: bytes):
+        if not self._lock.acquire(blocking=False):
+            return {"busy": True, "error": "BUSY"}
+        try:
+            return {"ok": True, "request": payload.hex()}
+        finally:
+            self._lock.release()