feat: add video quality selection for yt-dlp captures (#17)

- ytdlp::download() accepts quality: Option<&str>; quality_format()
  maps best/1080p/720p/480p/360p to yt-dlp -f format strings
- perform_capture() threads quality through to the downloader
- CaptureBody gains optional quality field; capture_handler validates
  it against the allowlist (400 on unknown values) before spawning
- CLI passes None (preserves existing best-quality behaviour)
- Frontend: isVideoSource() mirrors determine_source() exactly —
  shows quality picker only for yt-dlp-backed sources, excludes
  playlist/channel shorthands and tweet/thread paths
- submitCapture(archiveId, locator, quality) sends quality in POST body
- CSS: .capture-quality styles the inline select to fit the capture row
- Tests: quality_format unit tests in ytdlp.rs; two new route tests
  (valid quality accepted, invalid quality rejected with 400)
- Docs: video quality section added under Supported Platforms

frontend/src/api.js

@@ -95,11 +95,13 @@ export async function fetchTags(archiveId) {
   return getJson(`/api/archives/${archiveId}/tags`);
 }
 
-export async function submitCapture(archiveId, locator) {
+export async function submitCapture(archiveId, locator, quality = null) {
+  const payload = { locator }
+  if (quality && quality !== 'best') payload.quality = quality
   const res = await fetch(`/api/archives/${archiveId}/captures`, {
     method: "POST",
     headers: { "Content-Type": "application/json" },
-    body: JSON.stringify({ locator }),
+    body: JSON.stringify(payload),
   });
   if (!res.ok) {
     const body = await res.json().catch(() => ({}));
@@ -108,6 +110,12 @@ export async function submitCapture(archiveId, locator) {
   return res.json(); // { job_uid, status: "pending" }
 }
 
+// Returns { has_video: bool, qualities: string[] } e.g. { has_video: true, qualities: ["1080p","720p","480p"] }
+// Throws on network error; returns { has_video: false, qualities: [] } on non-video locators.
+export async function probeCapture(archiveId, locator) {
+  return getJson(`/api/archives/${archiveId}/captures/probe?locator=${encodeURIComponent(locator)}`);
+}
+
 export async function pollCaptureJob(archiveId, jobUid) {
   return getJson(`/api/archives/${archiveId}/capture_jobs/${jobUid}`);
 }