perf(workspace): share the startup inventory read and defer non-first-card IPC (J1)

Journey J1 / renderer.ipcCallsToFirstCard: 12 -> 7.

- PlaylistsService.getAllPlaylists() shares one in-flight SQLite read
  between concurrent callers (the playlist effect and the XMLTV source
  reconciliation at startup). Settled reads are never reused and every
  SQLite write detaches the pending read.
- getM3uFavoriteChannels() stops re-reading the write-once IndexedDB ->
  SQLite migration receipt once it has been seen.
- StartupDeferralService holds the download list, app update status and
  the dashboard's recent items and favorites until one task after the
  render that reveals the routed content (5 s safety timeout).
  IPTVNATOR_DISABLE_STARTUP_DEFERRAL=1 is the kill switch.
- reconcileEpgSources and the two distinct migration-flag reads stay on
  the critical path: the first is the #1548 revision fence, the second
  are different keys, not duplicates.

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

docs/development/electron-debugging.md

@@ -34,6 +34,7 @@ IPTVNATOR_TRACE_STARTUP=1 pnpm nx serve electron-backend
     - `IPTVNATOR_PERF_COUNT_SQL=1`, together with `IPTVNATOR_PERF_CAPTURE=1`, also counts every SQL statement of the main-process and database-worker connections for `main.sqlStatementsBeforeReadyToShow`; it wraps each statement execution, including every row of a bulk insert, so only the launch journey sets it and the import benchmarks leave it unset
     - `IPTVNATOR_PERF_WORKER_PROFILING=1` enables development/test-only, request-scoped worker receive/work/response-post timestamps, thread CPU, event-loop utilization/delay, count-only playlist serialization/SQLite write/read/deserialization plus Xtream category/content/cache-clear/delete/in-source-search phase events, profiling-only worker cancel-receipt acknowledgements, valid-sample-counted isolate peak memory, and the database worker's idle-only one-shot post-GC heap probe; overlapping database requests are explicitly invalidated instead of misattributed, the performance benchmark sets the flag automatically, and production launches must leave it unset
     - `IPTVNATOR_DISABLE_COMPILE_CACHE=1` disables the main-process V8 compile cache; `IPTVNATOR_COMPILE_CACHE_DIR=<dir>` relocates it. The startup trace reports the outcome as `compile-cache`
+    - `IPTVNATOR_DISABLE_STARTUP_DEFERRAL=1` starts the renderer's startup gate open, so the download list, update status and dashboard recent/favorites reads run before the first screen again (kill switch; see "Startup work before the first card" in `docs/architecture/performance-journeys.md`)
 
 - Settings, portal request/response, and trace payloads must use
   `@iptvnator/shared/logging` or the redacting portal logger before reaching