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.

Tests/ArchiveMessageBatchPolicyTests.swift

@@ -7,8 +7,9 @@ final class ArchiveMessageBatchPolicyTests: XCTestCase {
         XCTAssertGreaterThan(ArchiveMessageBatchPolicy.pageSize, 0)
     }
 
-    func testOnlyOneStanzaIsDecodedPerMainActorSlice() {
-        XCTAssertEqual(ArchiveMessageBatchPolicy.decodeSliceSize, 1)
+    func testStanzasAreDecodedInSmallBatchesPerMainActorSlice() {
+        XCTAssertGreaterThan(ArchiveMessageBatchPolicy.decodeSliceSize, 0)
+        XCTAssertLessThanOrEqual(ArchiveMessageBatchPolicy.decodeSliceSize, 16)
         XCTAssertGreaterThan(ArchiveMessageBatchPolicy.interSliceDelayNanoseconds, 0)
     }