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

CHANGELOG.md

@@ -22,6 +22,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Added
 
+- Achievement rank-up notifications: the client now shows rank-up popups when achievements level up, using per-character tracking of last-displayed levels ([#165](https://github.com/Mezeporta/Erupe/issues/165))
+- Database migration `0008_achievement_displayed_levels` (tracks last-displayed achievement levels)
 - Savedata corruption defense (tier 1): bounded decompression in nullcomp prevents OOM from crafted payloads, bounds-checked delta patching prevents buffer overflows, compressed payload size limits (512KB) and decompressed size limits (1MB) reject oversized saves, rotating savedata backups (3 slots, 30-minute interval) provide recovery points
 - Savedata corruption defense (tier 2): SHA-256 checksum on decompressed savedata verified on every load, atomic DB transactions wrapping character data + house data + hash + backup in a single commit, per-character save mutex preventing concurrent save races
 - Database migration `0007_savedata_integrity` (rotating backup table + integrity checksum column)