feat: Swagger API 문서 추가 + 보스레이드/플레이어 레벨 시스템

- swaggo/swag 기반 전체 API 엔드포인트 Swagger 어노테이션 (59개)
- /swagger/ 경로에 Swagger UI 제공
- 보스레이드 데디서버 관리 (등록, 하트비트, 슬롯 리셋)
- 플레이어 레벨/경험치 시스템 및 스탯 성장

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/player/level.go

@@ -0,0 +1,71 @@
+package player
+
+// MaxLevel is the maximum player level.
+const MaxLevel = 50
+
+// RequiredExp returns the experience needed to reach the next level.
+// Formula: level^2 * 100
+func RequiredExp(level int) int {
+	return level * level * 100
+}
+
+// CalcStatsForLevel computes combat stats for a given level.
+func CalcStatsForLevel(level int) (maxHP, maxMP, attackPower float64) {
+	maxHP = 100 + float64(level-1)*10
+	maxMP = 50 + float64(level-1)*5
+	attackPower = 10 + float64(level-1)*2
+	return
+}
+
+// LevelUpResult holds the result of applying experience gain.
+type LevelUpResult struct {
+	OldLevel   int     `json:"oldLevel"`
+	NewLevel   int     `json:"newLevel"`
+	Experience int     `json:"experience"`
+	NextExp    int     `json:"nextExp"`
+	MaxHP      float64 `json:"maxHp"`
+	MaxMP      float64 `json:"maxMp"`
+	AttackPower float64 `json:"attackPower"`
+	LeveledUp  bool    `json:"leveledUp"`
+}
+
+// ApplyExperience adds exp to current level/exp and applies level ups.
+// Returns the result including new stats if leveled up.
+func ApplyExperience(currentLevel, currentExp, gainedExp int) LevelUpResult {
+	level := currentLevel
+	exp := currentExp + gainedExp
+
+	// Process level ups
+	for level < MaxLevel {
+		required := RequiredExp(level)
+		if exp < required {
+			break
+		}
+		exp -= required
+		level++
+	}
+
+	// Cap at max level
+	if level >= MaxLevel {
+		level = MaxLevel
+		exp = 0 // No more exp needed at max level
+	}
+
+	maxHP, maxMP, attackPower := CalcStatsForLevel(level)
+
+	nextExp := 0
+	if level < MaxLevel {
+		nextExp = RequiredExp(level)
+	}
+
+	return LevelUpResult{
+		OldLevel:    currentLevel,
+		NewLevel:    level,
+		Experience:  exp,
+		NextExp:     nextExp,
+		MaxHP:       maxHP,
+		MaxMP:       maxMP,
+		AttackPower: attackPower,
+		LeveledUp:   level > currentLevel,
+	}
+}