fix(server-storage): the move uploads playlists create-only and merges into a copy moved first

Two tabs or devices moving browser copies with the same playlist id both
planned an upload; the browser documents carry no served version, so
both PUTs were unconditional and the later one overwrote the first copy
(and any change made on it since), with both moves reporting success.

The backend's playlist PUT now honours `If-None-Match: *` (412 when the
playlist exists; checked and written in one synchronous handler). The
move uploads with it through ServerPlaylistDocumentStore
.dbCreateAppPlaylist, and an upload that finds its id taken merges its
favorites and history into that copy instead, counted as merged.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/architecture/pwa-self-hosted.md

@@ -86,7 +86,8 @@ Server storage adds versioned routes under `/v1`, which the PWA uses in
   `header` fields (`409 payload_missing` without a stored row); every `PUT`
   answers with the new `ETag` and `X-Replaced-ETag`, the version it
   replaced, and one with `If-Match` answers `412 conflict` when another
-  write replaced that version
+  write replaced that version, one with `If-None-Match: *` when the
+  playlist exists already
 - `GET /v1/user-state`, `PUT|DELETE /v1/user-state/:namespace/:playlistId`:
   Xtream favorites, recently viewed, collection snapshots and playback
   positions, one JSON value per namespace and playlist; `GET` adds a
@@ -363,7 +364,8 @@ settings and guards read them in every build.
   a conditional mutation, a conflict retried at the next start). A
   playlist the server already has, by id or by source (Xtream server and
   user, Stalker portal and MAC, M3U URL), merges into the server's copy; the
-  others upload with their ids. A merged playlist gains the browser
+  others upload with their ids, create-only (`If-None-Match: *`): one that
+  another tab or device moved first is merged into instead of overwritten. A merged playlist gains the browser
   document's favorites and history (M3U favorites name channels by stream
   URL, as the app looks them up; older ones by a channel id that is random
   per import, which becomes its URL on both sides; history entries keep