feat: Sprint 3 — Prestige Loop endless

- Migration saves: saveVersion pattern + migrateSave lazy (v1→v2)
- Formule ADN rebalancée: log10 + clamp min 1 + cap bonus ×4
- Prestige Experience: modal fullscreen, preview ADN, stats run, best run
- Arbre V2: 25 nœuds, 3 capstones, post-capstones repeatables (scaling par tranche)
- Convergence évolutif Alpha→Omega (tier system)
- Reset arbre: 1 gratuit/prestige, payant linéaire au-delà
- Milestones prestige: 8 paliers (1→100), cosmétiques exclusifs, bonus gameplay
- balance.ts: constantes centralisées pour playtest
- 136 tests green, 0 regression

Backend/src/models/GameSaveManager.js

@@ -15,18 +15,20 @@ class GameSaveManager extends AbstractManager {
 
   async upsert(userId, gameState, metadata) {
     const { lifetimeTadpoles, prestigeCount, playTimeSeconds } = metadata;
+    const saveVersion = gameState.saveVersion ?? 1;
     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 (?, ?, ?, ?, ?)
+      `INSERT INTO ${this.table} (user_id, game_state, save_version, lifetime_tadpoles, prestige_count, play_time_seconds)
+       VALUES (?, ?, ?, ?, ?, ?)
        ON DUPLICATE KEY UPDATE
          game_state = VALUES(game_state),
+         save_version = VALUES(save_version),
          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]
+      [userId, gameStateJson, saveVersion, lifetimeTadpoles, prestigeCount, playTimeSeconds]
     );
 
     return result.affectedRows;