docs site: release the deploy lock after each deploy

The receiver took the deploy lock but never released it, which starved the
poller (flock -n failed instantly on every cycle).

publishing/webhook-receiver.py

@@ -107,7 +107,6 @@ def run(cmd, timeout, cwd=None):
 def deploy(commit_hint=""):
     """Pull main, rebuild, republish. Returns (ok, log_tail)."""
     try:
-        with open("/dev/null") as devnull:
         fcntl.flock(deploy_lock_file, fcntl.LOCK_EX | fcntl.LOCK_NB)
     except BlockingIOError:
         return False, "another deploy is already running; skipped"
@@ -133,6 +132,8 @@ def deploy(commit_hint=""):
         log.exception("deploy failed")
         tail.append(f"error: {exc}")
         return False, "\n".join(tail)
+    finally:
+        fcntl.flock(deploy_lock_file, fcntl.LOCK_UN)
 
 
 class Handler(BaseHTTPRequestHandler):