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/test_api.py

@@ -0,0 +1,20 @@
+from fastapi import FastAPI
+from pydantic import BaseModel
+import time
+
+app = FastAPI(title="Test API (no serial)")
+
+class Echo(BaseModel):
+    msg: str
+
+@app.get("/health")
+def health():
+    return {"ok": True, "ts": time.time()}
+
+@app.get("/ping")
+def ping():
+    return {"pong": True}
+
+@app.post("/echo")
+def echo(body: Echo):
+    return {"echo": body.msg}