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/database/migrations/002_game_saves.sql

@@ -0,0 +1,14 @@
+-- Migration 002 — Game saves with anti-cheat metadata
+-- Safe: CREATE TABLE IF NOT EXISTS, no data loss
+-- Run: mysql -u <user> -p clickerz < migrations/002_game_saves.sql
+
+CREATE TABLE IF NOT EXISTS game_saves (
+  id INT AUTO_INCREMENT PRIMARY KEY,
+  user_id INT NOT NULL UNIQUE,
+  game_state JSON NOT NULL,
+  last_save TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+  lifetime_tadpoles BIGINT DEFAULT 0,
+  prestige_count INT DEFAULT 0,
+  play_time_seconds INT DEFAULT 0,
+  FOREIGN KEY (user_id) REFERENCES users(id) ON DELETE CASCADE
+);