Check the proxy at startup instead of failing on first use

A proxy pointed at 127.0.0.1 from inside a container reaches the
container itself, and the only sign was a connection error buried in the
first search. Startup now probes the proxy over TCP and says what is
wrong, naming host.docker.internal when loopback was configured.

The README also covers the follow-up trap: even that address fails when
the proxy listens on loopback only, so it shows how to check the bind
address and what to change.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/sources/ytdlp.ts

@@ -1,4 +1,5 @@
 import { spawn } from "node:child_process";
+import { connect } from "node:net";
 import { randomUUID } from "node:crypto";
 import { constants } from "node:fs";
 import { access } from "node:fs/promises";
@@ -52,6 +53,40 @@ export function describeProxy(): string | null {
   }
 }
 
+/**
+ * A quick TCP probe of the proxy. The usual mistake is pointing a container at
+ * 127.0.0.1, which is the container itself — better to say so at startup than to
+ * let every search fail with a connection error.
+ */
+export function checkProxy(timeoutMs = 4000): Promise<string | null> {
+  if (!config.YTDLP_PROXY) return Promise.resolve(null);
+  let target: URL;
+  try {
+    target = new URL(config.YTDLP_PROXY);
+  } catch {
+    return Promise.resolve("YTDLP_PROXY не является корректным URL");
+  }
+  const port = Number(target.port) || (target.protocol === "https:" ? 443 : 1080);
+
+  return new Promise((resolve) => {
+    const socket = connect({ host: target.hostname, port });
+    const done = (result: string | null) => {
+      socket.destroy();
+      resolve(result);
+    };
+    socket.setTimeout(timeoutMs);
+    socket.once("connect", () => done(null));
+    socket.once("timeout", () => done(`прокси ${target.hostname}:${port} не отвечает`));
+    socket.once("error", (err: NodeJS.ErrnoException) => {
+      const hint =
+        target.hostname === "127.0.0.1" || target.hostname === "localhost"
+          ? " — внутри контейнера это сам контейнер; используйте host.docker.internal"
+          : "";
+      done(`прокси ${target.hostname}:${port} недоступен (${err.code ?? err.message})${hint}`);
+    });
+  });
+}
+
 export type CookieStatus = "ok" | "read-only" | "missing";
 
 /**