feat: Sprint 1 — backend fondations TetaRdPG

Auth SuperOAuth (JWT validation + httpOnly cookie), entités users/characters/level_thresholds,
lazy calculation endurance, seed 100 niveaux, config prod-ready (trust proxy, helmet, CORS, rate limit).
Validé : health 200, auth flow, character CRUD, endurance lazy, 401 sans cookie.

src/common/health.controller.ts

@@ -0,0 +1,9 @@
+import { Controller, Get } from '@nestjs/common';
+
+@Controller('health')
+export class HealthController {
+  @Get()
+  check() {
+    return { status: 'ok', timestamp: new Date().toISOString() };
+  }
+}