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_mocks_test.go

@@ -11,12 +11,16 @@ var errNotFound = errors.New("not found")
 // --- mockAchievementRepo ---
 
 type mockAchievementRepo struct {
-	scores        [33]int32
-	ensureCalled  bool
-	ensureErr     error
-	getScoresErr  error
-	incrementErr  error
-	incrementedID uint8
+	scores          [33]int32
+	ensureCalled    bool
+	ensureErr       error
+	getScoresErr    error
+	incrementErr    error
+	incrementedID   uint8
+	displayedLevels []byte
+	displayedErr    error
+	savedLevels     []byte
+	saveLevelsErr   error
 }
 
 func (m *mockAchievementRepo) EnsureExists(_ uint32) error {
@@ -33,6 +37,15 @@ func (m *mockAchievementRepo) IncrementScore(_ uint32, id uint8) error {
 	return m.incrementErr
 }
 
+func (m *mockAchievementRepo) GetDisplayedLevels(_ uint32) ([]byte, error) {
+	return m.displayedLevels, m.displayedErr
+}
+
+func (m *mockAchievementRepo) SaveDisplayedLevels(_ uint32, levels []byte) error {
+	m.savedLevels = levels
+	return m.saveLevelsErr
+}
+
 // --- mockMailRepo ---
 
 type mockMailRepo struct {