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/migrations/sql/0008_achievement_displayed_levels.sql

@@ -0,0 +1,4 @@
+-- 0008: Add displayed_levels to achievements table for rank-up notifications (#165).
+-- Stores 33 bytes (one level per achievement) representing the last level
+-- the client acknowledged seeing. NULL means never displayed (shows all rank-ups).
+ALTER TABLE public.achievements ADD COLUMN IF NOT EXISTS displayed_levels bytea;