feat: extract secrets to .env, remove hardcoded credentials

- Backend: .env file for DB/JWT/Redis/MinIO config, appsettings.json cleared
- Backend: Program.cs loads .env at startup (no extra NuGet packages)
- Frontend: .env files for VITE_API_URL, api-clients use import.meta.env
- Added vite-env.d.ts type declarations for both frontends
- All hardcoded localhost:5000 replaced with env variable
- Added .env.example template for onboarding

backend/.env.example

@@ -0,0 +1,15 @@
+# PostgreSQL
+ConnectionStrings__Default=Host=localhost;Port=5432;Database=health_manager;Username=postgres;Password=your_password
+
+# JWT
+Jwt__Secret=your-jwt-secret-change-me
+Jwt__Issuer=HealthManager
+Jwt__Audience=HealthManagerApp
+
+# Redis (reserved)
+Redis__Connection=localhost:6379
+
+# MinIO (reserved)
+MinIO__Endpoint=localhost:9000
+MinIO__AccessKey=minioadmin
+MinIO__SecretKey=minioadmin