refactor(matrix): rewrite adapter from matrix-nio to mautrix-python

Translate all nio SDK calls to mautrix equivalents while preserving the
adapter structure, business logic, and all features (E2EE, reactions,
threading, mention gating, text batching, media caching, voice MSC3245).

Key changes:
- nio.AsyncClient -> mautrix.client.Client + HTTPAPI + MemoryStateStore
- Manual E2EE key management -> OlmMachine with auto key lifecycle
- isinstance(resp, nio.XxxResponse) -> mautrix returns values directly
- add_event_callback per type -> single ROOM_MESSAGE handler with
  msgtype dispatch
- Room state (member_count, display_name) via async state store lookups
- Upload/download return ContentURI/bytes directly (no wrapper objects)

gateway/run.py

@@ -1724,7 +1724,7 @@ class GatewayRunner:
         elif platform == Platform.MATRIX:
             from gateway.platforms.matrix import MatrixAdapter, check_matrix_requirements
             if not check_matrix_requirements():
-                logger.warning("Matrix: matrix-nio not installed or credentials not set. Run: pip install 'matrix-nio[e2e]'")
+                logger.warning("Matrix: mautrix not installed or credentials not set. Run: pip install 'mautrix[encryption]'")
                 return None
             return MatrixAdapter(config)