feat: gate download options on real-time yt-dlp video analysis

Options (quality, subtitles) are now derived from a yt-dlp -J probe of
the submitted URL instead of a static list, so users can't pick
combinations the source video doesn't actually support. Submission is
blocked until the probe succeeds.

- New /api/probe endpoint + src/lib/ytdlp-probe.ts, with its own rate
  limiter (rate-limit.ts refactored into a createRateLimiter factory).
- New options: subtitle language selection, clip start/end trim, MP3
  audio quality.
- Fixed buildYtdlpArgs: format=mp3 never triggered audio extraction
  (-x/--audio-format), and quality=best never applied --merge-output-format,
  so the chosen container had no real effect.
- Cross-platform yt-dlp invocation: the bundled bin/yt-dlp is a Python
  zipapp relying on a shebang, which Windows' spawn() can't run
  directly. buildYtdlpCommand() runs it through `python` on Windows and
  execs it directly on Linux/o2switch, where the shebang works natively.

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

messages/fr.json

@@ -9,13 +9,23 @@
     "urlLabel": "URL de la vidéo",
     "format": "Format",
     "quality": "Qualité",
-    "subtitles": "Sous-titres (fr, en)",
+    "subtitles": "Sous-titres",
+    "subtitleLanguages": "Langues des sous-titres",
     "advanced": "Options avancées",
     "formats": "mp4 · mp3 · webm · mkv",
     "submit": "Télécharger",
     "submitting": "Envoi en cours...",
     "errorRateLimit": "Trop de soumissions. Réessayez dans une heure.",
-    "errorGeneric": "Erreur lors de la soumission."
+    "errorGeneric": "Erreur lors de la soumission.",
+    "analyzing": "Analyse de la vidéo...",
+    "probeError": "Impossible d'analyser cette vidéo. Vérifiez le lien ou réessayez.",
+    "retry": "Réessayer",
+    "audioOnlySource": "Source audio uniquement",
+    "clipRange": "Extrait (optionnel)",
+    "clipStart": "Début (s)",
+    "clipEnd": "Fin (s)",
+    "audioQuality": "Qualité audio",
+    "videoLength": "Durée : {duration}"
   },
   "status": {
     "heading": "Statut du téléchargement",