feat(sprint1-step3b): backend save system + anti-cheat + données rattrapées

- game_saves table + migration 002 (JSON state, anti-cheat metadata)
- saveControllers.js : load/save avec validation delta ressources (750k/s × 1.1)
- GameSaveManager : upsert MySQL ON DUPLICATE KEY UPDATE
- useSaveSync hook : auto-save 30s + keepalive beforeunload + guest fallback
- save-validation.test.ts : 8 tests anti-cheat
- economy.ts : arbre d'évolution 5 nœuds + prestige ADN (rattrapage step 2)
- economy.test.ts : +40 tests (évolution tree, multipliers, start bonus)
- GDD + SPRINT1.md : docs sprint complètes
- Rethème data : shop.json, Achievements.json, Cookie, Legal (rattrapage step 1)

Backend/src/models/GameSaveManager.js

@@ -0,0 +1,43 @@
+const AbstractManager = require("./AbstractManager");
+
+class GameSaveManager extends AbstractManager {
+  constructor() {
+    super({ table: "game_saves" });
+  }
+
+  async getByUserId(userId) {
+    const [rows] = await this.database.query(
+      `SELECT * FROM ${this.table} WHERE user_id = ?`,
+      [userId]
+    );
+    return rows[0] ?? null;
+  }
+
+  async upsert(userId, gameState, metadata) {
+    const { lifetimeTadpoles, prestigeCount, playTimeSeconds } = metadata;
+    const gameStateJson = JSON.stringify(gameState);
+
+    const [result] = await this.database.query(
+      `INSERT INTO ${this.table} (user_id, game_state, lifetime_tadpoles, prestige_count, play_time_seconds)
+       VALUES (?, ?, ?, ?, ?)
+       ON DUPLICATE KEY UPDATE
+         game_state = VALUES(game_state),
+         lifetime_tadpoles = VALUES(lifetime_tadpoles),
+         prestige_count = VALUES(prestige_count),
+         play_time_seconds = VALUES(play_time_seconds),
+         last_save = CURRENT_TIMESTAMP`,
+      [userId, gameStateJson, lifetimeTadpoles, prestigeCount, playTimeSeconds]
+    );
+
+    return result.affectedRows;
+  }
+
+  async delete(userId) {
+    await this.database.query(
+      `DELETE FROM ${this.table} WHERE user_id = ?`,
+      [userId]
+    );
+  }
+}
+
+module.exports = GameSaveManager;