[Server] Harden private chat history identity matching

Match both participants by account id and name so a federated user's remote id cannot collide with a local account, and stop storing foreign ids in the local chat log. Extract the paging assembly into a pure helper and cover the whole-second boundary rules with gtests.

servatrice/CMakeLists.txt

@@ -10,6 +10,7 @@ set(servatrice_SOURCES
     src/forwarded_address.cpp
     src/main.cpp
     src/metrics_registry.cpp
+    src/private_chat_paging.cpp
     src/ratelimiter.cpp
     src/servatrice.cpp
     src/servatrice_connection_pool.cpp