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/database/migrations/003_save_version.sql

@@ -0,0 +1,4 @@
+-- Migration 003: Add save_version column for Sprint 3 migration system
+-- Safe to run on existing data — defaults to 1 (Sprint 2 saves)
+
+ALTER TABLE game_saves ADD COLUMN save_version INT DEFAULT 1 AFTER game_state;

Backend/database/schema.sql

@@ -16,6 +16,7 @@ CREATE TABLE game_saves (
   id INT AUTO_INCREMENT PRIMARY KEY,
   user_id INT NOT NULL UNIQUE,
   game_state JSON NOT NULL,
+  save_version INT DEFAULT 1,
   last_save TIMESTAMP DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
   lifetime_tadpoles BIGINT DEFAULT 0,
   prestige_count INT DEFAULT 0,