backend: version + network endpoints, LAN-friendly CORS/origins

Add public GET /api/version (current version + GitHub-release update check,
cached, fail-soft) and GET /api/network (detected LAN addresses). Accept
localhost/private-LAN origins in CORS and Better Auth trusted origins via a
shared isAllowedOrigin helper, so staff can reach the app over the clinic
network. The seed-demo.ts fake test data is removed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/lib/origins.ts

@@ -0,0 +1,44 @@
+// Decides which browser origins may call the API with credentials.
+//
+// temetro is self-hosted: a clinic runs it on one machine and other departments
+// reach it over the LAN by the host's IP (e.g. http://192.168.1.20:3000). The
+// browser there sends that LAN origin, which must be allowed for both CORS and
+// Better Auth's CSRF/trusted-origin check. We allow:
+//   - FRONTEND_URL (the configured origin),
+//   - any explicitly listed TRUSTED_ORIGINS (or "*" for any),
+//   - localhost and private/LAN hosts (so LAN access works with no config).
+import { env } from "../env.js";
+
+const configured = (env.TRUSTED_ORIGINS ?? "")
+  .split(",")
+  .map((s) => s.trim())
+  .filter(Boolean);
+
+const allowAll = configured.includes("*");
+
+function isPrivateHost(hostname: string): boolean {
+  if (hostname === "localhost" || hostname === "127.0.0.1" || hostname === "::1") {
+    return true;
+  }
+  // Private IPv4 ranges (RFC 1918) + link-local.
+  if (/^10\./.test(hostname)) return true;
+  if (/^192\.168\./.test(hostname)) return true;
+  if (/^172\.(1[6-9]|2\d|3[01])\./.test(hostname)) return true;
+  if (/^169\.254\./.test(hostname)) return true;
+  // mDNS .local hostnames (e.g. clinic-pc.local).
+  if (hostname.endsWith(".local")) return true;
+  return false;
+}
+
+/** True if `origin` (an `Origin` header value) is allowed to call the API. */
+export function isAllowedOrigin(origin: string | undefined | null): boolean {
+  if (!origin) return false;
+  if (allowAll) return true;
+  if (origin === env.FRONTEND_URL) return true;
+  if (configured.includes(origin)) return true;
+  try {
+    return isPrivateHost(new URL(origin).hostname);
+  } catch {
+    return false;
+  }
+}