feat(sprint5): quest system + arcs + rebalance endurance/damage/xp

Quest system:
  4 entities (quest_arcs, quests, player_quests, player_quest_arcs)
  Arc "Les Marais du Têtard" (4 quêtes narratives)
  3 quêtes standalone répétables (chasse/forge/craft)
  5 achievements liés (quests_completed + quest_arc_completed)
  Event-driven: combat/forge/craft/loot émettent quest.progress
  API: available, active, completed, accept, claim, arcs

Rebalance:
  Endurance coût combat 10→5, regen 6min→3min (20/h), repos 20→10
  Dégâts joueur +3 base (plus de combats de 13 tours au level 1)
  Défaite endurance penalty 50→25
  XP monstres réduite (25→8 Têtard, 130→50 Golem) — quêtes = source principale

src/quest/player-quest-arc.entity.ts

@@ -0,0 +1,38 @@
+import {
+  Entity,
+  PrimaryGeneratedColumn,
+  Column,
+  ManyToOne,
+  JoinColumn,
+  Index,
+} from 'typeorm';
+import { Character } from '../character/entities/character.entity';
+import { QuestArc } from './quest-arc.entity';
+
+@Entity('player_quest_arcs')
+@Index(['characterId', 'questArcId'], { unique: true })
+export class PlayerQuestArc {
+  @PrimaryGeneratedColumn('uuid')
+  id: string;
+
+  @Column({ name: 'character_id' })
+  @Index()
+  characterId: string;
+
+  @ManyToOne(() => Character)
+  @JoinColumn({ name: 'character_id' })
+  character: Character;
+
+  @Column({ name: 'quest_arc_id' })
+  questArcId: string;
+
+  @ManyToOne(() => QuestArc)
+  @JoinColumn({ name: 'quest_arc_id' })
+  questArc: QuestArc;
+
+  @Column({ default: false })
+  completed: boolean;
+
+  @Column({ name: 'completed_at', type: 'timestamp', nullable: true })
+  completedAt: Date | null;
+}