video: fully decouple video server from music (no fallback + hard save guard)

Two real coupling bugs, fixed:
- resolve_video_server still fell back to the music active server when no explicit
  video pick was set, so changing the music server changed video. Removed: video
  now uses ONLY an explicit video pick or the configured server(s) (Plex default
  when both). Changing the music server never changes video.
- The shared settings page could persist active_media_server from the video side.
  Guarded saveSettings itself (not just the debounced auto-save) so it NEVER runs
  while data-side=video — video saves only via /api/video/*.

Test: video does not follow the music active server. One-way isolation, both ways.

core/video/sources.py

@@ -92,12 +92,10 @@ def resolve_video_server(db=None):
         return "plex"
     if pref == "jellyfin" and jelly_ok:
         return "jellyfin"
-
-    active = config_manager.get_active_media_server()
-    if active == "plex" and plex_ok:
-        return "plex"
-    if active == "jellyfin" and jelly_ok:
-        return "jellyfin"
+    # Fully INDEPENDENT of the music 'active server' — only an explicit video pick
+    # or the configured server(s) decide, so changing the music server NEVER
+    # changes video (and vice-versa). Auto-pick the single configured one; default
+    # to Plex when both are set (the user picks Jellyfin via the Video Source panel).
     if plex_ok and not jelly_ok:
         return "plex"
     if jelly_ok and not plex_ok:

tests/test_video_server_resolution.py

@@ -47,6 +47,15 @@ def test_independent_of_music_active(monkeypatch, vdb):
 
 def test_both_configured_default_then_explicit_pick(monkeypatch, vdb):
     _set_cm(monkeypatch, True, True, "plex")
-    assert resolve_video_server(vdb) == "plex"           # falls back to the active one
+    assert resolve_video_server(vdb) == "plex"           # both → Plex default
     vdb.set_setting("video_server", "jellyfin")
     assert resolve_video_server(vdb) == "jellyfin"       # explicit video pick wins
+
+
+def test_does_not_follow_music_active_server(monkeypatch, vdb):
+    # Both configured + music set to Jellyfin, but NO explicit video pick → video
+    # stays on Plex. Changing the music server must never change video.
+    _set_cm(monkeypatch, True, True, "jellyfin")
+    assert resolve_video_server(vdb) == "plex"
+    vdb.set_setting("video_server", "jellyfin")          # only an explicit pick switches video
+    assert resolve_video_server(vdb) == "jellyfin"

webui/static/settings.js

@@ -2867,6 +2867,11 @@ function _getTagConfig(path) {
 }
 
 async function saveSettings(quiet = false) {
+    // ISOLATION: this saves MUSIC settings (and persists active_media_server from
+    // the shared server toggle). The video side reuses this page but must never
+    // write music config — it saves via /api/video/* only. So never run here while
+    // on the video side, no matter what triggered it (auto-save OR manual).
+    if (document.body.getAttribute('data-side') === 'video') return;
     // Validate file organization templates before saving
     const validationErrors = validateFileOrganizationTemplates();
     if (validationErrors.length > 0) {