Eliminate the last three direct DB accesses from handler code:
- CharacterRepo.LoadSaveData: replaces db.Query in GetCharacterSaveData,
using QueryRow instead of Query+Next for cleaner single-row access
- EventRepo.GetEventQuests, UpdateEventQuestStartTime, BeginTx: moves
event quest enumeration and rotation queries behind the repo layer
- UserRepo.BanUser: consolidates permanent/temporary ban upserts into a
single method with nil/*time.Time semantics
Move 22 raw SQL queries from 4 handler files into dedicated repository
structs, continuing the repository extraction pattern. Achievement
insert uses ON CONFLICT DO NOTHING to eliminate check-then-insert race,
and IncrementScore validates the column index to prevent SQL injection.
