fix: buffer operations in Stale/StaleWithBuffer instead of throwing (#6342)

When 5+ users edit concurrently, revision gaps trigger the Stale state
which throws 'Ignored server-side change.' on incoming operations. This
causes document divergence, silent data corruption, and eventually
force-resets that discard local edits.

Changes:
- Stale/StaleWithBuffer.applyServer: buffer incoming operations instead
  of throwing, apply them sequentially after missing ops arrive
- Stale.applyClient: carry pendingOps to StaleWithBuffer on transition
- editor-socketio-server: retry-based force-reset (3 failures) instead
  of immediate nuclear reset on any OT error
- hex2rgb: defensive null checks for old non-hex color records

Signed-off-by: bxff <51504045+bxff@users.noreply.github.com>

public/docs/release-notes.md

@@ -1,5 +1,11 @@
 # Release Notes
 
+## <i class="fa fa-tag"></i> 1.x.x <i class="fa fa-calendar-o"></i> UNRELEASED
+
+### Bugfixes
+- Fix data loss when 5+ users edit a document concurrently, caused by the OT client discarding operations during revision gap recovery ([#6342](https://github.com/hedgedoc/hedgedoc/issues/6342))
+- Add defensive null checks to `hex2rgb` to prevent crashes from non-hex color values
+
 ## <i class="fa fa-tag"></i> 1.10.7 <i class="fa fa-calendar-o"></i> 2026-02-24
 
 ### Bugfixes