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__/rate-limit.test.ts

@@ -1,41 +1,53 @@
-import { isRateLimited } from '../rate-limit'
+import { createRateLimiter } from '../rate-limit'
 
 beforeEach(() => {
   jest.useFakeTimers()
-  jest.resetModules()
 })
 
 afterEach(() => {
   jest.useRealTimers()
 })
 
-describe('isRateLimited', () => {
+describe('createRateLimiter', () => {
   it('allows requests under the limit', () => {
+    const limiter = createRateLimiter(5, 3_600_000)
     const ip = '1.2.3.4'
     for (let i = 0; i < 5; i++) {
-      expect(isRateLimited(ip)).toBe(false)
+      expect(limiter.isLimited(ip)).toBe(false)
     }
   })
 
   it('blocks the 6th request within the window', () => {
+    const limiter = createRateLimiter(5, 3_600_000)
     const ip = '10.0.0.1'
-    for (let i = 0; i < 5; i++) isRateLimited(ip)
-    expect(isRateLimited(ip)).toBe(true)
+    for (let i = 0; i < 5; i++) limiter.isLimited(ip)
+    expect(limiter.isLimited(ip)).toBe(true)
   })
 
   it('resets after the window expires', () => {
+    const limiter = createRateLimiter(5, 3_600_000)
     const ip = '10.0.0.2'
-    for (let i = 0; i < 5; i++) isRateLimited(ip)
-    expect(isRateLimited(ip)).toBe(true)
+    for (let i = 0; i < 5; i++) limiter.isLimited(ip)
+    expect(limiter.isLimited(ip)).toBe(true)
     jest.advanceTimersByTime(3_600_001)
-    expect(isRateLimited(ip)).toBe(false)
+    expect(limiter.isLimited(ip)).toBe(false)
   })
 
   it('tracks different IPs independently', () => {
+    const limiter = createRateLimiter(5, 3_600_000)
     const ipA = '192.168.1.1'
     const ipB = '192.168.1.2'
-    for (let i = 0; i < 5; i++) isRateLimited(ipA)
-    expect(isRateLimited(ipA)).toBe(true)
-    expect(isRateLimited(ipB)).toBe(false)
+    for (let i = 0; i < 5; i++) limiter.isLimited(ipA)
+    expect(limiter.isLimited(ipA)).toBe(true)
+    expect(limiter.isLimited(ipB)).toBe(false)
+  })
+
+  it('tracks separate limiter instances independently', () => {
+    const limiterA = createRateLimiter(1, 3_600_000)
+    const limiterB = createRateLimiter(1, 3_600_000)
+    const ip = '5.5.5.5'
+    expect(limiterA.isLimited(ip)).toBe(false)
+    expect(limiterA.isLimited(ip)).toBe(true)
+    expect(limiterB.isLimited(ip)).toBe(false)
   })
 })