fix(server-storage): keep URL favorites, flush before moving, cache-full and cut-off bodies

- Moving M3U favorites keeps URL favorites and anything the server cannot
  resolve; only older channel ids are translated.
- Queued user-data edits are sent before the move writes combined values.
- A re-read that does not fit the cache keeps the old cache and baseline
  together.
- A response body cut off after its headers counts as offline.

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

docs/architecture/pwa-self-hosted.md

@@ -202,7 +202,9 @@ settings and guards read them in every build.
   (the page-hide flush waits for a batch still travelling), a failed write,
   including a refused `keepalive` one, stays queued for the next flush, and
   re-reads run one at a time and keep entries edited while they were in
-  flight or not yet saved. Before the first read finishes, an edit is
+  flight or not yet saved. A re-read whose result does not fit the cache
+  keeps the old cache and baseline together. A response body cut off after
+  its headers counts as offline. Before the first read finishes, an edit is
   compared with the cache, so it sends only its own entry. A re-read
   updates the cache; views already open show another device's changes on
   their next load (no live push). Start-up waits
@@ -230,9 +232,9 @@ settings and guards read them in every build.
   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
-  document's favorites and history (M3U channel ids are random per import,
-  so they are translated through the stream URL; unmatched ones are
-  dropped), and the legacy Xtream maps follow under
+  document's favorites and history (M3U favorites name channels by stream
+  URL; older ones by a channel id that is random per import and is
+  translated through the URL; nothing is dropped), and the legacy Xtream maps follow under
   the server id; `combineUserData` unions lists (playback positions by
   content type and Xtream id, renamed to the server playlist) and merges
   maps, the server's value winning on a conflict. Browser data is never deleted, and