A thumbnail Media cannot make is asked for less and less: each failure doubles the wait, 5 minutes up to a day (#555)

ddiff --git a/server/media-proxy/thumbnails.js b/server/media-proxy/thumbnails.js
iindex f77b61ba..f4641394 100644
---- a/server/media-proxy/thumbnails.js
++++ b/server/media-proxy/thumbnails.js
@@@ -70,6 +70,16 @@ function rememberNoPicture(key, ms = NO_PICTURE_MS) {
      if (noPicture.size >= 5000) noPicture.delete(noPicture.keys().next().value);
      noPicture.set(key, Date.now() + ms);
  }
++// Media's job failing again and again for the same recording (a file ffmpeg cannot draw a frame from) is not going to
++// change in five minutes: each failure doubles the wait, from 5 minutes up to a day, and a picture resets it (VOD 1803
++// failed 32 times in three hours on 2026-11-02, one Media job each).
++const failures = new Map();   // `${kind}:${id}` → failures in a row
++function failedAgain(key) {
++    const n = Math.min((failures.get(key) || 0) + 1, 12);
++    if (failures.size >= 5000 && !failures.has(key)) failures.delete(failures.keys().next().value);
++    failures.set(key, n);
++    return Math.min(MEDIA_BUSY_MS * 2 ** (n - 1), NO_VIDEO_MS);
++}
  
  async function generateFor(kind, id, req, res) {
      try {
@@@ -96,11 +106,11 @@ async function generateFor(kind, id, req, res) {
              if (!(err && err.name === 'MediaApiError' && (err.status === 404 || err.status === 429 || failed))) throw err;
              if (failed) console.warn(`[Thumbnails] Media's thumbnail job failed for ${key}: ${err.status}${err.body && err.body.job_id ? ` (${err.body.job_id})` : ''}`);
              const noVideo = err.status === 404 && err.body && err.body.code === 'media.no_video';
--            rememberNoPicture(key, noVideo ? NO_VIDEO_MS : err.status === 404 ? NO_PICTURE_MS : MEDIA_BUSY_MS);
++            rememberNoPicture(key, noVideo ? NO_VIDEO_MS : err.status === 404 ? NO_PICTURE_MS : failed ? failedAgain(key) : MEDIA_BUSY_MS);
              return res.json({ thumbnail_url: null });
          }
          const url = media.publicUrl(out?.url || meta.thumbnail_url);
--        if (url) return res.json({ thumbnail_url: url });
++        if (url) { failures.delete(key); return res.json({ thumbnail_url: url }); }
          // Media is still working on it (202 with its job): no picture yet, nothing failed.
          if (out && out.job) return res.status(202).json({ thumbnail_url: null });
          // Media answered with neither a picture nor a job (a recording it cannot draw a frame from yet): no picture,
@@@ -190,3 +200,4 @@ router.get('/:filename', async (req, res) => {
  });
  
  module.exports = router;
++module.exports._failedAgain = failedAgain;   // for test/thumbnail-backoff.test.js
ddiff --git a/test/thumbnail-no-picture.test.js b/test/thumbnail-no-picture.test.js
iindex eef4dec3..e2483ab7 100644
---- a/test/thumbnail-no-picture.test.js
++++ b/test/thumbnail-no-picture.test.js
@@@ -78,6 +78,29 @@ console.warn = () => {};
      const live = await post('/api/thumbnails/generate/vod/5833');
      assert.deepStrictEqual([live.status, live.json, jobs], [200, { thumbnail_url: null }, 0], 'a recording VOD asks Media for nothing');
  
++    // The same recording's job failing again and again (VOD 1803, 32 times in three hours on 2026-11-02): each failure
++    // doubles the wait, from 5 minutes up to a day; a picture resets it.
++    const backoff = require('../server/media-proxy/thumbnails')._failedAgain;
++    const waits = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10].map(() => backoff('vod:9999') / 60000);
++    assert.deepStrictEqual(waits, [5, 10, 20, 40, 80, 160, 320, 640, 1280, 1440], 'minutes: doubling, a day at most');
++    media.getVod = async (id) => ({ id: Number(id), user_id: 1, visibility: 'public', thumbnail_url: null });
++    let fails = 0;
++    media.generateThumbnail = async () => {
++        fails++;
++        const err = new Error('Failed to generate thumbnail');
++        Object.assign(err, { name: 'MediaApiError', status: 500, body: { error: 'Failed to generate thumbnail' } });
++        throw err;
++    };
++    const t0 = realNow();
++    Date.now = () => t0;
++    await post('/api/thumbnails/generate/vod/1803');
++    Date.now = () => t0 + 6 * 60 * 1000;    // 6 minutes: past the first 5-minute wait
++    await post('/api/thumbnails/generate/vod/1803');
++    Date.now = () => t0 + 12 * 60 * 1000;   // 12 minutes: within the second, 10-minute wait
++    await post('/api/thumbnails/generate/vod/1803');
++    assert.strictEqual(fails, 2, 'the second failure waits twice as long');
++    Date.now = realNow;
++
      server.close();
      console.log('thumbnail no-picture: all checks passed');
      process.exit(0);