feat: zone locking — progression par arcs narratifs + arcs Égouts/Désert

Zones verrouillées: marais toujours ouvert, égouts après arc Marais,
désert après arc Égouts. Filtrage backend sur monstres ET boutique.

Arc "Les Égouts de la Cité" (4 quêtes, lv4-7, boss Roi des Rats)
Arc "Les Sables Brûlants" (3 quêtes, lv8-12, boss Sphinx)

GET /api/monsters/zones — retourne les zones avec statut unlocked.
Combat page: monstres groupés par zone, zones lockées avec icône cadenas.
Boutique: items filtrés par zones débloquées (potions toujours visibles).

src/common/zone-access.ts

@@ -0,0 +1,44 @@
+import { Repository } from 'typeorm';
+import { PlayerQuestArc } from '../quest/player-quest-arc.entity';
+import { QuestArc } from '../quest/quest-arc.entity';
+
+// Zone unlock chain: each zone requires completing the previous zone's arc
+// marais → always open
+// egouts → requires "Les Marais du Têtard" arc completed
+// desert → requires the egouts arc completed
+const ZONE_ORDER = ['marais', 'egouts', 'desert'];
+
+export async function getUnlockedZones(
+  characterId: string,
+  arcRepo: Repository<QuestArc>,
+  playerArcRepo: Repository<PlayerQuestArc>,
+): Promise<string[]> {
+  const unlocked: string[] = ['marais']; // always accessible
+
+  // Get all completed arcs for this character
+  const completedArcs = await playerArcRepo.find({
+    where: { characterId, completed: true },
+    relations: ['questArc'],
+  });
+  const completedArcZones = new Set(completedArcs.map(pa => pa.questArc?.zone).filter(Boolean));
+
+  // Check zone chain: each zone unlocks the next
+  for (let i = 0; i < ZONE_ORDER.length - 1; i++) {
+    const currentZone = ZONE_ORDER[i];
+    const nextZone = ZONE_ORDER[i + 1];
+
+    // Find arc for current zone
+    const arc = await arcRepo.findOne({ where: { zone: currentZone } });
+    if (!arc) continue;
+
+    // If this zone's arc is completed, unlock the next zone
+    const isCompleted = completedArcs.some(pa => pa.questArcId === arc.id);
+    if (isCompleted) {
+      unlocked.push(nextZone);
+    } else {
+      break; // Can't skip zones
+    }
+  }
+
+  return unlocked;
+}