Fix Docker signup and document deploy, migrate, and rebuild flow.

Use FormData and server-side sign-in after register, trust proxy origins for reverse-proxy deploys, pass public URL build args in Docker, and clarify that git pull plus up -d --build is required to ship code and run pending migrations.

Co-authored-by: Cursor <cursoragent@cursor.com>

src/proxy.ts

@@ -1,10 +1,11 @@
 import { NextResponse } from "next/server";
 import type { NextRequest } from "next/server";
+import { envBoolean } from "~/lib/env-boolean";
 
 export function proxy(request: NextRequest) {
   const { pathname } = request.nextUrl;
 
-  if (pathname === "/auth/register" && process.env.DISABLE_SIGNUPS === "true") {
+  if (pathname === "/auth/register" && envBoolean(process.env.DISABLE_SIGNUPS)) {
     const signInUrl = new URL("/auth/signin", request.url);
     signInUrl.searchParams.set("signup", "disabled");
     return NextResponse.redirect(signInUrl);