perf(xmpp): fix slow MAM/MUC-MAM and move OMEMO decryption off the main

thread

- Replace O(n^2) origin-id/stanza-id lookups in AppModel with hash
  indexes
  and stop rebuilding the whole index on every unseen stanza-id, so
  archive
  application no longer grows quadratically with history size.
- Run OMEMO decryption on a serial background queue instead of the main
  actor, and decrypt archived stanzas in small batches with a shorter
  yield,
  keeping the UI responsive during large archive catch-ups.
- Show the "Синхронизация истории…" banner only for the one-page
  bootstrap
  window; incremental backlog catch-up continues silently in the
  background.
- Fix MUC-MAM: publish decoded group mutations at the end of room
  catch-up;
  they were previously accumulated and then silently dropped.
- Retry a failed catch-up pass automatically (bounded) and lengthen the
  query/apply timeouts so a single slow page no longer leaves history
  unloaded until the next app activation.

Sources/Shared/Models/ArchiveMessageBatchPolicy.swift

@@ -6,10 +6,14 @@ import Foundation
 enum ArchiveMessageBatchPolicy {
     static let pageSize = 24
     static let bootstrapMessageLimit = 40
-    static let decodeSliceSize = 1
+    /// Decrypt this many archived stanzas per main-actor slice before yielding.
+    /// One-by-one decryption turns a large MAM catch-up into hours of serialized
+    /// work; a small batch keeps frames mostly intact while cutting the number
+    /// of run-loop hand-offs by the same factor.
+    static let decodeSliceSize = 8
     static let maximumBufferedStanzas = 48
-    /// Leave roughly half of a 60 Hz frame between archived decryptions. This
-    /// keeps UIScrollView touch handling responsive even when OMEMO work is
-    /// unusually expensive on older devices.
-    static let interSliceDelayNanoseconds: UInt64 = 8_000_000
+    /// A short yield between archived decryption slices keeps touch handling
+    /// responsive without the 8 ms per single message that previously dominated
+    /// catch-up time.
+    static let interSliceDelayNanoseconds: UInt64 = 1_000_000
 }