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>

src/lib/__tests__/ytdlp.test.ts

@@ -6,6 +6,10 @@ const base = {
   format: 'mp4',
   quality: 'best',
   subtitles: false,
+  subtitleLangs: null,
+  clipStart: null,
+  clipEnd: null,
+  audioQuality: null,
   extraArgs: null,
 }
 
@@ -22,23 +26,89 @@ describe('buildYtdlpArgs', () => {
     expect(args[idx + 1]).toContain('%(ext)s')
   })
 
-  it('adds format filter when quality is not "best"', () => {
-    const args = buildYtdlpArgs({ ...base, quality: '1080p' })
-    expect(args).toContain('-f')
+  it('always sets --merge-output-format to the chosen video format', () => {
+    const args = buildYtdlpArgs(base)
+    const idx = args.indexOf('--merge-output-format')
+    expect(idx).toBeGreaterThan(-1)
+    expect(args[idx + 1]).toBe('mp4')
   })
 
-  it('does not add -f flag when quality is "best"', () => {
-    expect(buildYtdlpArgs(base)).not.toContain('-f')
+  it('adds a height filter to -f when quality is not "best"', () => {
+    const args = buildYtdlpArgs({ ...base, quality: '1080p' })
+    const idx = args.indexOf('-f')
+    expect(idx).toBeGreaterThan(-1)
+    expect(args[idx + 1]).toContain('height<=?1080')
+  })
+
+  it('does not add a height filter when quality is "best"', () => {
+    const args = buildYtdlpArgs(base)
+    const idx = args.indexOf('-f')
+    expect(args[idx + 1]).not.toContain('height<=?')
+  })
+
+  it('extracts audio with -x and --audio-format when format is mp3', () => {
+    const args = buildYtdlpArgs({ ...base, format: 'mp3' })
+    expect(args).toContain('-x')
+    const idx = args.indexOf('--audio-format')
+    expect(args[idx + 1]).toBe('mp3')
+    expect(args).not.toContain('--merge-output-format')
+  })
+
+  it('sets --audio-quality to 0 for best mp3 quality', () => {
+    const args = buildYtdlpArgs({ ...base, format: 'mp3', audioQuality: 'best' })
+    const idx = args.indexOf('--audio-quality')
+    expect(args[idx + 1]).toBe('0')
+  })
+
+  it('sets --audio-quality to a bitrate when a specific mp3 quality is given', () => {
+    const args = buildYtdlpArgs({ ...base, format: 'mp3', audioQuality: '192' })
+    const idx = args.indexOf('--audio-quality')
+    expect(args[idx + 1]).toBe('192K')
   })
 
   it('adds subtitle flags when subtitles is true', () => {
     const args = buildYtdlpArgs({ ...base, subtitles: true })
-    expect(args).toContain('--write-sub')
+    expect(args).toContain('--write-subs')
+    expect(args).toContain('--write-auto-subs')
     expect(args).toContain('--sub-lang')
   })
 
   it('does not add subtitle flags when subtitles is false', () => {
-    expect(buildYtdlpArgs(base)).not.toContain('--write-sub')
+    expect(buildYtdlpArgs(base)).not.toContain('--write-subs')
+  })
+
+  it('defaults subtitle languages to fr,en when none are given', () => {
+    const args = buildYtdlpArgs({ ...base, subtitles: true })
+    const idx = args.indexOf('--sub-lang')
+    expect(args[idx + 1]).toBe('fr,en')
+  })
+
+  it('uses the given subtitle languages when provided', () => {
+    const args = buildYtdlpArgs({ ...base, subtitles: true, subtitleLangs: ['es', 'it'] })
+    const idx = args.indexOf('--sub-lang')
+    expect(args[idx + 1]).toBe('es,it')
+  })
+
+  it('adds --download-sections when a clip range is given', () => {
+    const args = buildYtdlpArgs({ ...base, clipStart: 10, clipEnd: 30 })
+    const idx = args.indexOf('--download-sections')
+    expect(args[idx + 1]).toBe('*10-30')
+  })
+
+  it('defaults clip start to 0 when only clipEnd is given', () => {
+    const args = buildYtdlpArgs({ ...base, clipEnd: 30 })
+    const idx = args.indexOf('--download-sections')
+    expect(args[idx + 1]).toBe('*0-30')
+  })
+
+  it('leaves the clip end open when only clipStart is given', () => {
+    const args = buildYtdlpArgs({ ...base, clipStart: 10 })
+    const idx = args.indexOf('--download-sections')
+    expect(args[idx + 1]).toBe('*10-')
+  })
+
+  it('does not add --download-sections when no clip range is given', () => {
+    expect(buildYtdlpArgs(base)).not.toContain('--download-sections')
   })
 
   it('appends extra args from a JSON array string', () => {