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/es.json

@@ -9,13 +9,23 @@
     "urlLabel": "URL del vídeo",
     "format": "Formato",
     "quality": "Calidad",
-    "subtitles": "Subtítulos (fr, en)",
+    "subtitles": "Subtítulos",
+    "subtitleLanguages": "Idiomas de los subtítulos",
     "advanced": "Opciones avanzadas",
     "formats": "mp4 · mp3 · webm · mkv",
     "submit": "Descargar",
     "submitting": "Enviando...",
     "errorRateLimit": "Demasiadas peticiones. Inténtalo de nuevo en una hora.",
-    "errorGeneric": "Error al enviar."
+    "errorGeneric": "Error al enviar.",
+    "analyzing": "Analizando el vídeo...",
+    "probeError": "No se pudo analizar este vídeo. Comprueba el enlace o inténtalo de nuevo.",
+    "retry": "Reintentar",
+    "audioOnlySource": "Fuente solo de audio",
+    "clipRange": "Fragmento (opcional)",
+    "clipStart": "Inicio (s)",
+    "clipEnd": "Fin (s)",
+    "audioQuality": "Calidad de audio",
+    "videoLength": "Duración: {duration}"
   },
   "status": {
     "heading": "Estado de la descarga",