[Server] Recursive alt search with match evidence (#7510)

* [Server] Recursive alt search with match evidence

getUserAlts() now walks a breadth-first graph to the requested depth (clamped 1-3): each level expands frontier accounts over registration email, current client id, recent session IPs (6-month window), and historic session client ids, so identifier changes no longer hide older alts. Every row carries match_reason (email / clientid:<id> / ip:<addr>), depth, and shared_count fan-out so moderators can distinguish real alts from VPN/shared-network overlap. Command_GetUserAlts depth is plumbed through; unset depth defaults to 1, preserving previous single-hop behavior for old clients.

* [Server] Fix alt search review findings

- Seed the BFS with the stored user name so the searched account is not
  re-listed as its own alt at depth 1.
- Normalise emails to lower case for the membership and fan-out checks;
  SQL compares them case-insensitively.
- Widen the per-level row limit by the visited count so level 2+ cannot
  return only already-known accounts.
- Prepare dynamic IN-list queries locally instead of through the shared
  prepared-statement cache, whose keys are query texts.
- Count historic client-id fan-out from _sessions, not from the current
  _users ids.
- Order capped identifiers by relevance (current email/client id first,
  then most recent session IPs) instead of alphabetically.

* Apply suggestion from @ZeldaZach

Co-authored-by: Zach H <zahalpern+github@gmail.com>

* Apply suggestion from @ZeldaZach

Co-authored-by: Zach H <zahalpern+github@gmail.com>

---------

Co-authored-by: Lukas Brübach <Bruebach.Lukas@bdosecurity.de>
Co-authored-by: Zach H <zahalpern+github@gmail.com>

servatrice/src/servatrice_database_interface.h

@@ -224,7 +224,7 @@ public:
                                                        const QString &beforeTime = QString(),
                                                        bool *hasMore = nullptr);
     QList<ServerInfo_UserSession> getUserSessions(const QString &userName, int limit);
-    QList<ServerInfo_UserAlt> getUserAlts(const QString &userName);
+    QList<ServerInfo_UserAlt> getUserAlts(const QString &userName, int depth = 1);
     QList<ServerInfo_ModeratorLogin> getModeratorLastLogins();
 
     // Uptime snapshot as recorded by Servatrice::statusUpdate() into the