feat(achievement): add rank-up notifications (#165)

RE'd putDisplayed_achievement from ZZ client DLL via Ghidra: the packet
sends opcode + 1 zero byte with no achievement ID, acting as a blanket
"I saw everything" signal.

Server changes:
- Track per-character last-displayed levels in new displayed_levels
  column (migration 0008)
- GetAchievement compares current vs displayed levels per entry
- DisplayedAchievement snapshots current levels to clear notifications
- Repo, service, mock, and 3 new service tests

Protbot changes:
- New --action achievement: fetches achievements, shows rank-up markers,
  sends DISPLAYED_ACHIEVEMENT, re-fetches to verify notifications clear
- Packet builders for GET/ADD/DISPLAYED_ACHIEVEMENT

server/channelserver/repo_interfaces.go

@@ -290,6 +290,8 @@ type AchievementRepo interface {
 	EnsureExists(charID uint32) error
 	GetAllScores(charID uint32) ([33]int32, error)
 	IncrementScore(charID uint32, achievementID uint8) error
+	GetDisplayedLevels(charID uint32) ([]byte, error)
+	SaveDisplayedLevels(charID uint32, levels []byte) error
 }
 
 // ShopRepo defines the contract for shop data access.