Add HMAC auth, user permissions, snake_case rename

Each request is signed with HMAC-SHA256 over timestamp+body using a
per-user secret loaded from a --secrets file (never env vars or git).
Users have a canApprove list controlling who may approve queued actions.
Queue entries track submitted_by for permission checks on approve/deny.

Also renames all identifiers to snake_case throughout the codebase.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/secrets.mjs

@@ -0,0 +1,23 @@
+import { readFileSync } from "fs";
+
+export function load_secrets(file_path) {
+	if (!file_path) {
+		throw new Error("--secrets <path> is required");
+	}
+	let raw;
+	try {
+		raw = readFileSync(file_path, "utf8");
+	} catch (err) {
+		throw new Error(`Cannot read secrets file at ${file_path}: ${err.message}`);
+	}
+	let parsed;
+	try {
+		parsed = JSON.parse(raw);
+	} catch (err) {
+		throw new Error(`Secrets file is not valid JSON: ${err.message}`);
+	}
+	if (!parsed.users || typeof parsed.users !== "object") {
+		throw new Error("Secrets file must have a top-level 'users' object");
+	}
+	return parsed;
+}