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/main.py

@@ -0,0 +1,83 @@
+import os
+import time
+import socket
+from fastapi import FastAPI, HTTPException
+from pydantic import BaseModel
+
+from .serial_bridge import SerialBridge, SerialConfig
+
+app = FastAPI(title="Pont bascule API", version="1.3")
+
+TRAME_ESCLAVE = b"\x01\x0D\x0A"
+TRAME_DSD = b"\x01\x10\x39\x39\x4D\x0D\x0A"
+
+
+def env(name: str, default: str) -> str:
+    return os.getenv(name, default)
+
+
+def hex2b(s: str) -> bytes:
+    s = s.strip().replace("0x", "").replace(",", " ")
+    parts = [p for p in s.split() if p]
+    return bytes(int(p, 16) for p in parts)
+
+
+cfg = SerialConfig(
+    port=env("SERIAL_PORT", "/dev/ttyUSB0"),
+    baudrate=int(env("SERIAL_BAUDRATE", "9600")),
+    timeout_s=float(env("SERIAL_TIMEOUT_S", "1.0")),
+    open_delay_s=float(env("SERIAL_OPEN_DELAY_S", "2.0")),       # ✅ comme Tkinter
+    post_write_delay_s=float(env("SERIAL_POST_WRITE_DELAY_S", "0.5")),  # ✅
+)
+bridge = SerialBridge(cfg)
+
+
+class CustomReq(BaseModel):
+    hex: str
+
+
+@app.get("/health")
+def health():
+    return {
+        "ok": True,
+        "mode": "serial",
+        "busy": bridge.busy(),
+        "hostname": socket.gethostname(),
+        "timestamp": time.time(),
+        "port": cfg.port,
+        "baudrate": cfg.baudrate,
+    }
+
+
+@app.get("/last")
+def last():
+    return bridge.last()
+
+
+@app.post("/send/esclave")
+def send_esclave():
+    res = bridge.send_and_read_once(TRAME_ESCLAVE)
+    if res.get("busy"):
+        raise HTTPException(status_code=409, detail=res["error"])
+    return res
+
+
+@app.post("/send/dsd")
+def send_dsd():
+    res = bridge.send_and_read_once(TRAME_DSD)
+    if res.get("busy"):
+        raise HTTPException(status_code=409, detail=res["error"])
+    return res
+
+
+@app.post("/send/custom")
+def send_custom(req: CustomReq):
+    try:
+        payload = hex2b(req.hex)
+    except Exception as e:
+        raise HTTPException(status_code=400, detail=f"Invalid hex: {e}")
+
+    res = bridge.send_and_read_once(payload)
+    if res.get("busy"):
+        raise HTTPException(status_code=409, detail=res["error"])
+    return res