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/mock_bridge.py

@@ -0,0 +1,18 @@
+import threading
+import time
+
+class MockBridge:
+    def __init__(self):
+        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:
+            time.sleep(0.1)
+            return {"ok": True, "response": "OK"}
+        finally:
+            self._lock.release()