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/app/api/downloads/__tests__/post.test.ts

@@ -1,14 +1,16 @@
 import { POST } from '../route'
 import { prisma } from '@/lib/prisma'
-import * as rateLimit from '@/lib/rate-limit'
+import { downloadRateLimiter } from '@/lib/rate-limit'
 
 jest.mock('@/lib/prisma', () => ({
   prisma: { download: { create: jest.fn() } },
 }))
-jest.mock('@/lib/rate-limit')
+jest.mock('@/lib/rate-limit', () => ({
+  downloadRateLimiter: { isLimited: jest.fn() },
+}))
 
 const mockCreate = prisma.download.create as jest.Mock
-const mockIsRateLimited = rateLimit.isRateLimited as jest.Mock
+const mockIsLimited = downloadRateLimiter.isLimited as jest.Mock
 
 function req(body: object, ip = '1.2.3.4') {
   return new Request('http://localhost/api/downloads', {
@@ -20,12 +22,12 @@ function req(body: object, ip = '1.2.3.4') {
 
 describe('POST /api/downloads', () => {
   beforeEach(() => {
-    mockIsRateLimited.mockReturnValue(false)
+    mockIsLimited.mockReturnValue(false)
     mockCreate.mockResolvedValue({ uuid: 'test-uuid' })
   })
 
   it('returns 429 when rate limited', async () => {
-    mockIsRateLimited.mockReturnValue(true)
+    mockIsLimited.mockReturnValue(true)
     const res = await POST(req({ url: 'https://y.com', format: 'mp4', quality: 'best', subtitles: false }))
     expect(res.status).toBe(429)
   })
@@ -40,6 +42,14 @@ describe('POST /api/downloads', () => {
     expect(res.status).toBe(400)
   })
 
+  it('returns 400 when clipEnd is not greater than clipStart', async () => {
+    const res = await POST(req({
+      url: 'https://y.com', format: 'mp4', quality: 'best', subtitles: false,
+      clipStart: 30, clipEnd: 10,
+    }))
+    expect(res.status).toBe(400)
+  })
+
   it('returns 201 with uuid on success', async () => {
     const res = await POST(req({ url: 'https://youtube.com/watch?v=abc', format: 'mp4', quality: 'best', subtitles: false }))
     expect(res.status).toBe(201)
@@ -58,4 +68,24 @@ describe('POST /api/downloads', () => {
       }),
     })
   })
+
+  it('stores subtitleLangs as a comma-joined string', async () => {
+    await POST(req({
+      url: 'https://y.com', format: 'mp4', quality: 'best', subtitles: true,
+      subtitleLangs: ['fr', 'en'],
+    }))
+    expect(mockCreate).toHaveBeenCalledWith({
+      data: expect.objectContaining({ subtitleLangs: 'fr,en' }),
+    })
+  })
+
+  it('stores clip range and audio quality when provided', async () => {
+    await POST(req({
+      url: 'https://y.com', format: 'mp3', quality: 'best', subtitles: false,
+      clipStart: 10, clipEnd: 30, audioQuality: '192',
+    }))
+    expect(mockCreate).toHaveBeenCalledWith({
+      data: expect.objectContaining({ clipStart: 10, clipEnd: 30, audioQuality: '192' }),
+    })
+  })
 })