feat: update Docker configuration and environment handling

- Added PostgreSQL service to docker-compose.yml with health checks and persistent storage.
- Updated bot service to use environment variable substitution for database connection string.
- Upgraded Go version in Dockerfile to 1.26.1-alpine.
- Modified config loading to ignore missing .env file for better compatibility with Docker.

internal/config/config.go

@@ -18,9 +18,8 @@ type Config struct {
 }
 
 func LoadConfig() (*Config, error) {
-	if err := godotenv.Load(); err != nil {
-		return nil, err
-	}
+	// Load .env if present (e.g. local dev). Ignore if missing (e.g. Docker injects env vars).
+	_ = godotenv.Load()
 
 	token := os.Getenv("BOT_TOKEN")
 	if token == "" {