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/it.json

@@ -9,13 +9,23 @@
     "urlLabel": "URL del video",
     "format": "Formato",
     "quality": "Qualità",
-    "subtitles": "Sottotitoli (fr, en)",
+    "subtitles": "Sottotitoli",
+    "subtitleLanguages": "Lingue dei sottotitoli",
     "advanced": "Opzioni avanzate",
     "formats": "mp4 · mp3 · webm · mkv",
     "submit": "Scarica",
     "submitting": "Invio in corso...",
     "errorRateLimit": "Troppe richieste. Riprova tra un'ora.",
-    "errorGeneric": "Errore durante l'invio."
+    "errorGeneric": "Errore durante l'invio.",
+    "analyzing": "Analisi del video...",
+    "probeError": "Impossibile analizzare questo video. Controlla il link o riprova.",
+    "retry": "Riprova",
+    "audioOnlySource": "Sorgente solo audio",
+    "clipRange": "Estratto (opzionale)",
+    "clipStart": "Inizio (s)",
+    "clipEnd": "Fine (s)",
+    "audioQuality": "Qualità audio",
+    "videoLength": "Durata: {duration}"
   },
   "status": {
     "heading": "Stato del download",