Module split: extract audio I/O helpers to server/audio.js

• getAudioDuration(path)         — ffprobe wrapper, returns seconds | null
  • splitAudioFile(in, dir, secs)  — ffmpeg -acodec copy chunking
  • downloadPodcastAudio(url, dst) — streams HTTP audio to disk

Also moved fetchUrl into util.js (alongside the other stateless
helpers) — it's a generic HTTP-GET-with-redirects used by RSS parsing
and channel discovery, not strictly audio.

server/index.js: 2758 → 2694 lines.

Smoke tested: server boots; /api/license-status, /api/health, /
respond. No behavior change.

server/audio.js

@@ -0,0 +1,86 @@
+// Audio I/O helpers — ffprobe for metadata, ffmpeg for splitting, plus
+// HTTP downloading for podcast episodes. Pure module: no state, no
+// Express, only takes paths/URLs and returns data.
+
+import { execFile } from "child_process";
+import { promisify } from "util";
+import path from "path";
+import http from "http";
+import https from "https";
+import { createWriteStream } from "fs";
+
+const execFileAsync = promisify(execFile);
+
+// ── Audio duration via ffprobe ──────────────────────────────────────────────
+// Returns the duration in seconds, or null if ffprobe can't read the file
+// (corrupt, missing, unsupported format). Caller decides what to do with
+// null — most call sites treat it as "unknown" and skip duration-dependent
+// branches.
+export async function getAudioDuration(filePath) {
+  try {
+    const { stdout } = await execFileAsync("ffprobe", [
+      "-v", "error",
+      "-show_entries", "format=duration",
+      "-of", "default=noprint_wrappers=1:nokey=1",
+      filePath,
+    ], { timeout: 15000 });
+    const dur = parseFloat(stdout.trim());
+    return isNaN(dur) ? null : dur;
+  } catch {
+    return null;
+  }
+}
+
+// ── Split a long audio file into chunks ─────────────────────────────────────
+// Used when a video is too long for a single Gemini transcription call.
+// Returns null if no split is needed (audio fits in one chunk), otherwise
+// an array of `{ path, startOffset, index }`. Uses `-acodec copy` so it's
+// fast and lossless — no re-encoding.
+export async function splitAudioFile(inputPath, outputDir, chunkSeconds = 2700) {
+  const duration = await getAudioDuration(inputPath);
+  if (!duration || duration <= chunkSeconds) return null;
+
+  const chunks = [];
+  let startSec = 0;
+  let i = 0;
+  while (startSec < duration) {
+    const chunkPath = path.join(outputDir, `chunk_${i}.mp3`);
+    const segLen = Math.min(chunkSeconds, duration - startSec);
+    await execFileAsync("ffmpeg", [
+      "-y", "-i", inputPath,
+      "-ss", String(startSec),
+      "-t", String(segLen),
+      "-acodec", "copy",
+      chunkPath,
+    ], { timeout: 120000 });
+    chunks.push({ path: chunkPath, startOffset: startSec, index: i });
+    startSec += chunkSeconds;
+    i++;
+  }
+  return chunks;
+}
+
+// ── Download a podcast episode by URL ───────────────────────────────────────
+// Streams the HTTP response straight to disk. Follows redirects. Rejects
+// on any non-200 final status. Used by /api/process when the input URL is
+// a podcast episode rather than a YouTube video.
+export function downloadPodcastAudio(audioUrl, destPath) {
+  return new Promise((resolve, reject) => {
+    const doFetch = (url) => {
+      const getter = url.startsWith("https") ? https : http;
+      getter.get(url, (res) => {
+        if (res.statusCode >= 300 && res.statusCode < 400 && res.headers.location) {
+          return doFetch(res.headers.location);
+        }
+        if (res.statusCode !== 200) {
+          return reject(new Error(`HTTP ${res.statusCode} downloading podcast audio`));
+        }
+        const fileStream = createWriteStream(destPath);
+        res.pipe(fileStream);
+        fileStream.on("finish", () => fileStream.close(resolve));
+        fileStream.on("error", reject);
+      }).on("error", reject);
+    };
+    doFetch(audioUrl);
+  });
+}