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>

config/app.config.ts

@@ -7,5 +7,8 @@ export const config = {
   STORAGE_PATH: process.env.STORAGE_PATH ?? '',
   RATE_LIMIT_MAX: 5,
   RATE_LIMIT_WINDOW_MS: 3_600_000,
+  PROBE_RATE_LIMIT_MAX: 20,
+  PROBE_RATE_LIMIT_WINDOW_MS: 3_600_000,
+  PROBE_TIMEOUT_MS: 20_000,
   BIN_DIR: process.env.BIN_DIR ?? path.join(process.cwd(), 'bin'),
 } as const