Wire playback resume end to end (task ka)

- Add JSON tags to model structs so API responses use camelCase keys (e.g., id, file_name, position_seconds).
- MediaDetail now includes progress; add ResumeFrom() helper on MediaDetail.
- Frontend playSelected() fetches /api/media/:id and reads progress.position_seconds
  before starting playback, then passes resume position to selectAndPlay().
- player.js selectAndPlay/loadMedia accept resumeFrom parameter instead of reading
  media.resume_from from list items.
- Backend test coverage: handlers_test verifies detail endpoint returns progress
  position in JSON; media_test verifies GetMediaDetail includes progress and
  ResumeFrom() value.
- Frontend test coverage: added web/js/tests/playback-resume.test.js validating
  detail JSON shape, resume position computation, and list item contract.

1 files changed, 9 insertions, 2 deletions

diff --git a/web/js/app.js b/web/js/app.js
index 8e8393e..2238e5b 100644
--- a/web/js/app.js
+++ b/web/js/app.js
@@ -243,12 +243,19 @@ function renderItem(m, index) {
   `;
 }
 
-function playSelected() {
+async function playSelected() {
   const el = currentElement();
   if (!el) return;
   const idx = parseInt(el.dataset.index, 10);
   const media = state.media[idx];
-  if (media) selectAndPlay(media, idx);
+  if (!media) return;
+  try {
+    const detail = await API.mediaDetail(media.id);
+    const resumeFrom = detail?.progress?.position_seconds ?? 0;
+    selectAndPlay(media, idx, resumeFrom);
+  } catch {
+    selectAndPlay(media, idx, 0);
+  }
 }
 
 async function shareSelected() {